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In classical mathematics, analytic geometry, also known as coordinate geometry or Cartesian geometry, is the study of
geometry Geometry (; ) is, with arithmetic, one of the oldest branches of mathematics. It is concerned with properties of space such as the distance, shape, size, and relative position of figures. A mathematician who works in the field of geometry is c ...
using a coordinate system. This contrasts with synthetic geometry. Analytic geometry is used in
physics Physics is the natural science that studies matter, its fundamental constituents, its motion and behavior through space and time, and the related entities of energy and force. "Physical science is that department of knowledge which ...
and
engineering Engineering is the use of scientific principles to design and build machines, structures, and other items, including bridges, tunnels, roads, vehicles, and buildings. The discipline of engineering encompasses a broad range of more speciali ...
, and also in
aviation Aviation includes the activities surrounding mechanical flight and the aircraft industry. ''Aircraft'' includes airplane, fixed-wing and helicopter, rotary-wing types, morphable wings, wing-less lifting bodies, as well as aerostat, lighter- ...
, rocketry, space science, and
spaceflight Spaceflight (or space flight) is an application of astronautics to fly spacecraft into or through outer space, either with or without humans on board. Most spaceflight is uncrewed and conducted mainly with spacecraft such as satellites in ...
. It is the foundation of most modern fields of geometry, including
algebraic Algebraic may refer to any subject related to algebra in mathematics and related branches like algebraic number theory and algebraic topology. The word algebra itself has several meanings. Algebraic may also refer to: * Algebraic data type, a data ...
, differential, discrete and computational geometry. Usually the Cartesian coordinate system is applied to manipulate equations for planes, straight lines, and circles, often in two and sometimes three dimensions. Geometrically, one studies the Euclidean plane (two dimensions) and Euclidean space. As taught in school books, analytic geometry can be explained more simply: it is concerned with defining and representing geometric shapes in a numerical way and extracting numerical information from shapes' numerical definitions and representations. That the algebra of the real numbers can be employed to yield results about the linear continuum of geometry relies on the Cantor–Dedekind axiom.


History


Ancient Greece

The Greek mathematician
Menaechmus :''There is also a Menaechmus in Plautus' play, ''The Menaechmi''.'' Menaechmus ( el, Μέναιχμος, 380–320 BC) was an ancient Greek mathematician, geometer and philosopher born in Alopeconnesus or Prokonnesos in the Thracian Chersones ...
solved problems and proved theorems by using a method that had a strong resemblance to the use of coordinates and it has sometimes been maintained that he had introduced analytic geometry. Apollonius of Perga, in '' On Determinate Section'', dealt with problems in a manner that may be called an analytic geometry of one dimension; with the question of finding points on a line that were in a ratio to the others. Apollonius in the ''Conics'' further developed a method that is so similar to analytic geometry that his work is sometimes thought to have anticipated the work of Descartes by some 1800 years. His application of reference lines, a diameter and a tangent is essentially no different from our modern use of a coordinate frame, where the distances measured along the diameter from the point of tangency are the abscissas, and the segments parallel to the tangent and intercepted between the axis and the curve are the ordinates. He further developed relations between the abscissas and the corresponding ordinates that are equivalent to rhetorical equations (expressed in words) of curves. However, although Apollonius came close to developing analytic geometry, he did not manage to do so since he did not take into account negative magnitudes and in every case the coordinate system was superimposed upon a given curve ''a posteriori'' instead of ''a priori''. That is, equations were determined by curves, but curves were not determined by equations. Coordinates, variables, and equations were subsidiary notions applied to a specific geometric situation.


Persia

The 11th-century Persian mathematician Omar Khayyam saw a strong relationship between geometry and algebra and was moving in the right direction when he helped close the gap between numerical and geometric algebra with his geometric solution of the general cubic equations, but the decisive step came later with Descartes. Omar Khayyam is credited with identifying the foundations of algebraic geometry, and his book ''Treatise on Demonstrations of Problems of Algebra'' (1070), which laid down the principles of analytic geometry, is part of the body of Persian mathematics that was eventually transmitted to Europe. Because of his thoroughgoing geometrical approach to algebraic equations, Khayyam can be considered a precursor to Descartes in the invention of analytic geometry.Cooper, G. (2003). Journal of the American Oriental Society,123(1), 248-249.


Western Europe

Analytic geometry was independently invented by
René Descartes René Descartes ( or ; ; Latinized: Renatus Cartesius; 31 March 1596 – 11 February 1650) was a French philosopher, scientist, and mathematician, widely considered a seminal figure in the emergence of modern philosophy and science. Ma ...
and
Pierre de Fermat Pierre de Fermat (; between 31 October and 6 December 1607 – 12 January 1665) was a French mathematician who is given credit for early developments that led to infinitesimal calculus, including his technique of adequality. In particular, he ...
, although Descartes is sometimes given sole credit. ''Cartesian geometry'', the alternative term used for analytic geometry, is named after Descartes. Descartes made significant progress with the methods in an essay titled ''
La Géométrie ''La Géométrie'' was published in 1637 as an appendix to ''Discours de la méthode'' ('' Discourse on the Method''), written by René Descartes. In the ''Discourse'', he presents his method for obtaining clarity on any subject. ''La Géométr ...
(Geometry)'', one of the three accompanying essays (appendices) published in 1637 together with his ''Discourse on the Method for Rightly Directing One's Reason and Searching for Truth in the Sciences'', commonly referred to as '' Discourse on Method''. ''La Geometrie'', written in his native
French French (french: français(e), link=no) may refer to: * Something of, from, or related to France ** French language, which originated in France, and its various dialects and accents ** French people, a nation and ethnic group identified with Franc ...
tongue, and its philosophical principles, provided a foundation for
calculus Calculus, originally called infinitesimal calculus or "the calculus of infinitesimals", is the mathematics, mathematical study of continuous change, in the same way that geometry is the study of shape, and algebra is the study of generalizati ...
in Europe. Initially the work was not well received, due, in part, to the many gaps in arguments and complicated equations. Only after the translation into
Latin Latin (, or , ) is a classical language belonging to the Italic languages, Italic branch of the Indo-European languages. Latin was originally a dialect spoken in the lower Tiber area (then known as Latium) around present-day Rome, but through ...
and the addition of commentary by van Schooten in 1649 (and further work thereafter) did Descartes's masterpiece receive due recognition. Pierre de Fermat also pioneered the development of analytic geometry. Although not published in his lifetime, a manuscript form of ''Ad locos planos et solidos isagoge'' (Introduction to Plane and Solid Loci) was circulating in Paris in 1637, just prior to the publication of Descartes' ''Discourse''. Clearly written and well received, the ''Introduction'' also laid the groundwork for analytical geometry. The key difference between Fermat's and Descartes' treatments is a matter of viewpoint: Fermat always started with an algebraic equation and then described the geometric curve that satisfied it, whereas Descartes started with geometric curves and produced their equations as one of several properties of the curves. As a consequence of this approach, Descartes had to deal with more complicated equations and he had to develop the methods to work with polynomial equations of higher degree. It was
Leonhard Euler Leonhard Euler ( , ; 15 April 170718 September 1783) was a Swiss mathematician, physicist, astronomer, geographer, logician and engineer who founded the studies of graph theory and topology and made pioneering and influential discoveries ...
who first applied the coordinate method in a systematic study of space curves and surfaces.


Coordinates

In analytic geometry, the plane is given a coordinate system, by which every point has a pair of
real number In mathematics, a real number is a number that can be used to measure a ''continuous'' one-dimensional quantity such as a distance, duration or temperature. Here, ''continuous'' means that values can have arbitrarily small variations. Every ...
coordinates. Similarly,
Euclidean space Euclidean space is the fundamental space of geometry, intended to represent physical space. Originally, that is, in Euclid's ''Elements'', it was the three-dimensional space of Euclidean geometry, but in modern mathematics there are Euclidea ...
is given coordinates where every point has three coordinates. The value of the coordinates depends on the choice of the initial point of origin. There are a variety of coordinate systems used, but the most common are the following: Stewart, James (2008). ''Calculus: Early Transcendentals'', 6th ed., Brooks Cole Cengage Learning.


Cartesian coordinates (in a plane or space)

The most common coordinate system to use is the Cartesian coordinate system, where each point has an ''x''-coordinate representing its horizontal position, and a ''y''-coordinate representing its vertical position. These are typically written as an ordered pair (''x'', ''y''). This system can also be used for three-dimensional geometry, where every point in
Euclidean space Euclidean space is the fundamental space of geometry, intended to represent physical space. Originally, that is, in Euclid's ''Elements'', it was the three-dimensional space of Euclidean geometry, but in modern mathematics there are Euclidea ...
is represented by an ordered triple of coordinates (''x'', ''y'', ''z'').


Polar coordinates (in a plane)

In
polar coordinates In mathematics, the polar coordinate system is a two-dimensional coordinate system in which each point on a plane is determined by a distance from a reference point and an angle from a reference direction. The reference point (analogous to th ...
, every point of the plane is represented by its distance ''r'' from the origin and its
angle In Euclidean geometry, an angle is the figure formed by two rays, called the '' sides'' of the angle, sharing a common endpoint, called the ''vertex'' of the angle. Angles formed by two rays lie in the plane that contains the rays. Angles ...
''θ'', with ''θ'' normally measured counterclockwise from the positive ''x''-axis. Using this notation, points are typically written as an ordered pair (''r'', ''θ''). One may transform back and forth between two-dimensional Cartesian and polar coordinates by using these formulae: x = r\, \cos\theta,\, y = r\, \sin\theta; \, r = \sqrt,\, \theta = \arctan(y/x). This system may be generalized to three-dimensional space through the use of cylindrical or spherical coordinates.


Cylindrical coordinates (in a space)

In cylindrical coordinates, every point of space is represented by its height ''z'', its
radius In classical geometry, a radius (plural, : radii) of a circle or sphere is any of the line segments from its Centre (geometry), center to its perimeter, and in more modern usage, it is also their length. The name comes from the latin ''radius'', ...
''r'' from the ''z''-axis and the
angle In Euclidean geometry, an angle is the figure formed by two rays, called the '' sides'' of the angle, sharing a common endpoint, called the ''vertex'' of the angle. Angles formed by two rays lie in the plane that contains the rays. Angles ...
''θ'' its projection on the ''xy''-plane makes with respect to the horizontal axis.


Spherical coordinates (in a space)

In spherical coordinates, every point in space is represented by its distance ''ρ'' from the origin, the
angle In Euclidean geometry, an angle is the figure formed by two rays, called the '' sides'' of the angle, sharing a common endpoint, called the ''vertex'' of the angle. Angles formed by two rays lie in the plane that contains the rays. Angles ...
''θ'' its projection on the ''xy''-plane makes with respect to the horizontal axis, and the angle ''φ'' that it makes with respect to the ''z''-axis. The names of the angles are often reversed in physics.


Equations and curves

In analytic geometry, any equation involving the coordinates specifies a
subset In mathematics, set ''A'' is a subset of a set ''B'' if all elements of ''A'' are also elements of ''B''; ''B'' is then a superset of ''A''. It is possible for ''A'' and ''B'' to be equal; if they are unequal, then ''A'' is a proper subset of ...
of the plane, namely the
solution set In mathematics, a solution set is the set of values that satisfy a given set of equations or inequalities. For example, for a set of polynomials over a ring , the solution set is the subset of on which the polynomials all vanish (evaluate t ...
for the equation, or locus. For example, the equation ''y'' = ''x'' corresponds to the set of all the points on the plane whose ''x''-coordinate and ''y''-coordinate are equal. These points form a line, and ''y'' = ''x'' is said to be the equation for this line. In general, linear equations involving ''x'' and ''y'' specify lines,
quadratic equation In algebra, a quadratic equation () is any equation that can be rearranged in standard form as ax^2 + bx + c = 0\,, where represents an unknown value, and , , and represent known numbers, where . (If and then the equation is linear, not qu ...
s specify
conic section In mathematics, a conic section, quadratic curve or conic is a curve obtained as the intersection of the surface of a cone with a plane. The three types of conic section are the hyperbola, the parabola, and the ellipse; the circle is a ...
s, and more complicated equations describe more complicated figures.Percey Franklyn Smith, Arthur Sullivan Gale (1905)''Introduction to Analytic Geometry'', Athaeneum Press Usually, a single equation corresponds to a curve on the plane. This is not always the case: the trivial equation ''x'' = ''x'' specifies the entire plane, and the equation ''x''2 + ''y''2 = 0 specifies only the single point (0, 0). In three dimensions, a single equation usually gives a surface, and a curve must be specified as the intersection of two surfaces (see below), or as a system of parametric equations. The equation ''x''2 + ''y''2 = ''r''2 is the equation for any circle centered at the origin (0, 0) with a radius of r.


Lines and planes

Lines in a Cartesian plane, or more generally, in affine coordinates, can be described algebraically by ''linear'' equations. In two dimensions, the equation for non-vertical lines is often given in the '' slope-intercept form'': y = mx + b where: * ''m'' is the
slope In mathematics, the slope or gradient of a line is a number that describes both the ''direction'' and the ''steepness'' of the line. Slope is often denoted by the letter ''m''; there is no clear answer to the question why the letter ''m'' is use ...
or
gradient In vector calculus, the gradient of a scalar-valued differentiable function of several variables is the vector field (or vector-valued function) \nabla f whose value at a point p is the "direction and rate of fastest increase". If the gr ...
of the line. * ''b'' is the y-intercept of the line. * ''x'' is the independent variable of the function ''y'' = ''f''(''x''). In a manner analogous to the way lines in a two-dimensional space are described using a point-slope form for their equations, planes in a three dimensional space have a natural description using a point in the plane and a vector orthogonal to it (the normal vector) to indicate its "inclination". Specifically, let \mathbf_0 be the position vector of some point P_0 = (x_0,y_0,z_0), and let \mathbf = (a,b,c) be a nonzero vector. The plane determined by this point and vector consists of those points P, with position vector \mathbf, such that the vector drawn from P_0 to P is perpendicular to \mathbf. Recalling that two vectors are perpendicular if and only if their dot product is zero, it follows that the desired plane can be described as the set of all points \mathbf such that \mathbf \cdot (\mathbf-\mathbf_0) =0. (The dot here means a
dot product In mathematics, the dot product or scalar productThe term ''scalar product'' means literally "product with a scalar as a result". It is also used sometimes for other symmetric bilinear forms, for example in a pseudo-Euclidean space. is an alg ...
, not scalar multiplication.) Expanded this becomes a (x-x_0)+ b(y-y_0)+ c(z-z_0)=0, This is just a linear equation: ax + by + cz + d = 0, \text d = -(ax_0 + by_0 + cz_0). Conversely, it is easily shown that if ''a'', ''b'', ''c'' and ''d'' are constants and ''a'', ''b'', and ''c'' are not all zero, then the graph of the equation ax + by + cz + d = 0, This familiar equation for a plane is called the ''general form'' of the equation of the plane. In three dimensions, lines can ''not'' be described by a single linear equation, so they are frequently described by parametric equations: x = x_0 + at y = y_0 + bt z = z_0 + ct where: * ''x'', ''y'', and ''z'' are all functions of the independent variable ''t'' which ranges over the real numbers. * (''x''0, ''y''0, ''z''0) is any point on the line. * ''a'', ''b'', and ''c'' are related to the slope of the line, such that the vector (''a'', ''b'', ''c'') is parallel to the line.


Conic sections

In the Cartesian coordinate system, the graph of a
quadratic equation In algebra, a quadratic equation () is any equation that can be rearranged in standard form as ax^2 + bx + c = 0\,, where represents an unknown value, and , , and represent known numbers, where . (If and then the equation is linear, not qu ...
in two variables is always a conic section – though it may be degenerate, and all conic sections arise in this way. The equation will be of the form Ax^2 + Bxy + Cy^2 +Dx + Ey + F = 0\textA, B, C\text As scaling all six constants yields the same locus of zeros, one can consider conics as points in the five-dimensional projective space \mathbf^5. The conic sections described by this equation can be classified using the discriminant B^2 - 4AC . If the conic is non-degenerate, then: * if B^2 - 4AC < 0 , the equation represents an ellipse; ** if A = C and B = 0 , the equation represents a
circle A circle is a shape consisting of all points in a plane that are at a given distance from a given point, the centre. Equivalently, it is the curve traced out by a point that moves in a plane so that its distance from a given point is cons ...
, which is a special case of an ellipse; * if B^2 - 4AC = 0 , the equation represents a
parabola In mathematics, a parabola is a plane curve which is mirror-symmetrical and is approximately U-shaped. It fits several superficially different mathematical descriptions, which can all be proved to define exactly the same curves. One descri ...
; * if B^2 - 4AC > 0 , the equation represents a
hyperbola In mathematics, a hyperbola (; pl. hyperbolas or hyperbolae ; adj. hyperbolic ) is a type of smooth curve lying in a plane, defined by its geometric properties or by equations for which it is the solution set. A hyperbola has two pieces, ca ...
; ** if we also have A + C = 0 , the equation represents a rectangular hyperbola.


Quadric surfaces

A quadric, or quadric surface, is a ''2''-dimensional surface in 3-dimensional space defined as the locus of zeros of a quadratic polynomial. In coordinates , the general quadric is defined by the algebraic equationSilvio Lev
Quadrics
in "Geometry Formulas and Facts", excerpted from 30th Edition of ''CRC Standard Mathematical Tables and Formulas'', CRC Press, from The Geometry Center at University of Minnesota
\sum_^ x_i Q_ x_j + \sum_^ P_i x_i + R = 0. Quadric surfaces include
ellipsoid An ellipsoid is a surface that may be obtained from a sphere by deforming it by means of directional scalings, or more generally, of an affine transformation. An ellipsoid is a quadric surface;  that is, a surface that may be defined as th ...
s (including the
sphere A sphere () is a geometrical object that is a three-dimensional analogue to a two-dimensional circle. A sphere is the set of points that are all at the same distance from a given point in three-dimensional space.. That given point is the c ...
), paraboloids, hyperboloids,
cylinder A cylinder (from ) has traditionally been a three-dimensional solid, one of the most basic of curvilinear geometric shapes. In elementary geometry, it is considered a prism with a circle as its base. A cylinder may also be defined as an ...
s, cones, and planes.


Distance and angle

In analytic geometry, geometric notions such as
distance Distance is a numerical or occasionally qualitative measurement of how far apart objects or points are. In physics or everyday usage, distance may refer to a physical length or an estimation based on other criteria (e.g. "two counties over"). ...
and
angle In Euclidean geometry, an angle is the figure formed by two rays, called the '' sides'' of the angle, sharing a common endpoint, called the ''vertex'' of the angle. Angles formed by two rays lie in the plane that contains the rays. Angles ...
measure are defined using formulas. These definitions are designed to be consistent with the underlying Euclidean geometry. For example, using Cartesian coordinates on the plane, the distance between two points (''x''1, ''y''1) and (''x''2, ''y''2) is defined by the formula d = \sqrt, which can be viewed as a version of the Pythagorean theorem. Similarly, the angle that a line makes with the horizontal can be defined by the formula \theta = \arctan(m), where ''m'' is the
slope In mathematics, the slope or gradient of a line is a number that describes both the ''direction'' and the ''steepness'' of the line. Slope is often denoted by the letter ''m''; there is no clear answer to the question why the letter ''m'' is use ...
of the line. In three dimensions, distance is given by the generalization of the Pythagorean theorem: d = \sqrt, while the angle between two vectors is given by the
dot product In mathematics, the dot product or scalar productThe term ''scalar product'' means literally "product with a scalar as a result". It is also used sometimes for other symmetric bilinear forms, for example in a pseudo-Euclidean space. is an alg ...
. The dot product of two Euclidean vectors A and B is defined by \mathbf A\cdot\mathbf B \stackrel \left\, \mathbf A\right\, \left\, \mathbf B\right\, \cos\theta, where ''θ'' is the
angle In Euclidean geometry, an angle is the figure formed by two rays, called the '' sides'' of the angle, sharing a common endpoint, called the ''vertex'' of the angle. Angles formed by two rays lie in the plane that contains the rays. Angles ...
between A and B.


Transformations

Transformations are applied to a parent function to turn it into a new function with similar characteristics. The graph of R(x,y) is changed by standard transformations as follows: *Changing x to x-h moves the graph to the right h units. *Changing y to y-k moves the graph up k units. *Changing x to x/b stretches the graph horizontally by a factor of b. (think of the x as being dilated) *Changing y to y/a stretches the graph vertically. *Changing x to x\cos A+ y\sin A and changing y to -x\sin A + y\cos A rotates the graph by an angle A. There are other standard transformation not typically studied in elementary analytic geometry because the transformations change the shape of objects in ways not usually considered. Skewing is an example of a transformation not usually considered. For more information, consult the Wikipedia article on affine transformations. For example, the parent function y=1/x has a horizontal and a vertical asymptote, and occupies the first and third quadrant, and all of its transformed forms have one horizontal and vertical asymptote, and occupies either the 1st and 3rd or 2nd and 4th quadrant. In general, if y=f(x), then it can be transformed into y=af(b(x-k))+h. In the new transformed function, a is the factor that vertically stretches the function if it is greater than 1 or vertically compresses the function if it is less than 1, and for negative a values, the function is reflected in the x-axis. The b value compresses the graph of the function horizontally if greater than 1 and stretches the function horizontally if less than 1, and like a, reflects the function in the y-axis when it is negative. The k and h values introduce translations, h, vertical, and k horizontal. Positive h and k values mean the function is translated to the positive end of its axis and negative meaning translation towards the negative end. Transformations can be applied to any geometric equation whether or not the equation represents a function. Transformations can be considered as individual transactions or in combinations. Suppose that R(x,y) is a relation in the xy plane. For example, x^2+y^2-1=0 is the relation that describes the unit circle.


Finding intersections of geometric objects

For two geometric objects P and Q represented by the relations P(x,y) and Q(x,y) the intersection is the collection of all points (x,y) which are in both relations.While this discussion is limited to the xy-plane, it can easily be extended to higher dimensions. For example, P might be the circle with radius 1 and center (0,0): P = \ and Q might be the circle with radius 1 and center (1,0): Q = \. The intersection of these two circles is the collection of points which make both equations true. Does the point (0,0) make both equations true? Using (0,0) for (x,y), the equation for Q becomes (0-1)^2+0^2=1 or (-1)^2=1 which is true, so (0,0) is in the relation Q. On the other hand, still using (0,0) for (x,y) the equation for P becomes 0^2+0^2=1 or 0=1 which is false. (0,0) is not in P so it is not in the intersection. The intersection of P and Q can be found by solving the simultaneous equations: x^2+y^2 = 1 (x-1)^2+y^2 = 1. Traditional methods for finding intersections include substitution and elimination. Substitution: Solve the first equation for y in terms of x and then substitute the expression for y into the second equation: x^2+y^2 = 1 y^2=1-x^2. We then substitute this value for y^2 into the other equation and proceed to solve for x: (x-1)^2+(1-x^2)=1 x^2 -2x +1 +1 -x^2 =1 -2x = -1 x=1/2. Next, we place this value of x in either of the original equations and solve for y: (1/2)^2+y^2 = 1 y^2 =3/4 y = \frac. So our intersection has two points: \left(1/2,\frac\right) \;\; \text \;\; \left(1/2,\frac\right). Elimination: Add (or subtract) a multiple of one equation to the other equation so that one of the variables is eliminated. For our current example, if we subtract the first equation from the second we get (x-1)^2-x^2=0. The y^2 in the first equation is subtracted from the y^2 in the second equation leaving no y term. The variable y has been eliminated. We then solve the remaining equation for x, in the same way as in the substitution method: x^2 -2x +1 +1 -x^2 =1 -2x = -1 x=1/2. We then place this value of x in either of the original equations and solve for y: (1/2)^2+y^2 = 1 y^2 = 3/4 y = \frac. So our intersection has two points: \left(1/2,\frac\right) \;\; \text \;\; \left(1/2,\frac\right). For conic sections, as many as 4 points might be in the intersection.


Finding intercepts

One type of intersection which is widely studied is the intersection of a geometric object with the x and y coordinate axes. The intersection of a geometric object and the y-axis is called the y-intercept of the object. The intersection of a geometric object and the x-axis is called the x-intercept of the object. For the line y=mx+b, the parameter b specifies the point where the line crosses the y axis. Depending on the context, either b or the point (0,b) is called the y-intercept.


Geometric axis

Axis in geometry is the perpendicular line to any line, object or a surface. Also for this may be used the common language use as a: normal (prependicular) line, otherwise in engineering as ''axial line''. In
geometry Geometry (; ) is, with arithmetic, one of the oldest branches of mathematics. It is concerned with properties of space such as the distance, shape, size, and relative position of figures. A mathematician who works in the field of geometry is c ...
, a normal is an object such as a line or vector that is
perpendicular In elementary geometry, two geometric objects are perpendicular if they intersect at a right angle (90 degrees or π/2 radians). The condition of perpendicularity may be represented graphically using the '' perpendicular symbol'', ⟂. It c ...
to a given object. For example, in the two-dimensional case, the normal line to a curve at a given point is the line perpendicular to the tangent line to the curve at the point. In the three-dimensional case a surface normal, or simply normal, to a surface at a point ''P'' is a vector that is
perpendicular In elementary geometry, two geometric objects are perpendicular if they intersect at a right angle (90 degrees or π/2 radians). The condition of perpendicularity may be represented graphically using the '' perpendicular symbol'', ⟂. It c ...
to the tangent plane to that surface at ''P''. The word "normal" is also used as an adjective: a line normal to a plane, the normal component of a
force In physics, a force is an influence that can change the motion of an object. A force can cause an object with mass to change its velocity (e.g. moving from a state of rest), i.e., to accelerate. Force can also be described intuitively as a ...
, the normal vector, etc. The concept of normality generalizes to orthogonality.


Spherical and nonlinear planes and their tangents

Tangent is the linear approximation of a spherical or other curved or twisted line of a function.


Tangent lines and planes

In
geometry Geometry (; ) is, with arithmetic, one of the oldest branches of mathematics. It is concerned with properties of space such as the distance, shape, size, and relative position of figures. A mathematician who works in the field of geometry is c ...
, the tangent line (or simply tangent) to a plane curve at a given point is the straight line that "just touches" the curve at that point. Informally, it is a line through a pair of infinitely close points on the curve. More precisely, a straight line is said to be a tangent of a curve at a point on the curve if the line passes through the point on the curve and has slope where ''f'' is the
derivative In mathematics, the derivative of a function of a real variable measures the sensitivity to change of the function value (output value) with respect to a change in its argument (input value). Derivatives are a fundamental tool of calculus. ...
of ''f''. A similar definition applies to
space curve In mathematics, a curve (also called a curved line in older texts) is an object similar to a line, but that does not have to be straight. Intuitively, a curve may be thought of as the trace left by a moving point. This is the definition that ...
s and curves in ''n''-dimensional
Euclidean space Euclidean space is the fundamental space of geometry, intended to represent physical space. Originally, that is, in Euclid's ''Elements'', it was the three-dimensional space of Euclidean geometry, but in modern mathematics there are Euclidea ...
. As it passes through the point where the tangent line and the curve meet, called the point of tangency, the tangent line is "going in the same direction" as the curve, and is thus the best straight-line approximation to the curve at that point. Similarly, the tangent plane to a surface at a given point is the plane that "just touches" the surface at that point. The concept of a tangent is one of the most fundamental notions in
differential geometry Differential geometry is a mathematical discipline that studies the geometry of smooth shapes and smooth spaces, otherwise known as smooth manifolds. It uses the techniques of differential calculus, integral calculus, linear algebra and mult ...
and has been extensively generalized; see Tangent space.


See also

* Applied geometry *
Cross product In mathematics, the cross product or vector product (occasionally directed area product, to emphasize its geometric significance) is a binary operation on two vectors in a three-dimensional oriented Euclidean vector space (named here E), and ...
* Rotation of axes * Translation of axes *
Vector space In mathematics and physics, a vector space (also called a linear space) is a set whose elements, often called '' vectors'', may be added together and multiplied ("scaled") by numbers called ''scalars''. Scalars are often real numbers, but can ...


Notes


References


Books

* * * John Casey (1885
Analytic Geometry of the Point, Line, Circle, and Conic Sections
link from
Internet Archive The Internet Archive is an American digital library with the stated mission of "universal access to all knowledge". It provides free public access to collections of digitized materials, including websites, software applications/games, music, ...
. * *


Articles

* * * * *


External links


Coordinate Geometry topics
with interactive animations {{Authority control