Quadratic formula

In elementary algebra, the quadratic formula is a formula that provides the solution(s) to a quadratic equation. There are other ways of solving a quadratic equation instead of using the quadratic formula, such as factoring (direct factoring, grouping, AC method), completing the square, graphing and others.

Given a general quadratic equation of the form

:$ax^2+bx+c=0$

with representing an unknown, with , and representing constants, and with , the quadratic formula is:

:$x = \frac$

where the plus–minus symbol "±" indicates that the quadratic equation has two solutions. Written separately, they become:

:$x_1=\frac\quad\text\quad x_2=\frac$

Each of these two solutions is also called a root (or zero) of the quadratic equation. Geometrically, these roots represent the -values at which ''any'' parabola, explicitly given as , crosses the -axis.

As well as being a formula that yields the zeros of any parabola, the quadratic formula can also be used to identify the axis of symmetry of the parabola, and the number of real zeros the quadratic equation contains.

The expression ''b''² − 4''ac'' is known as discriminant. If then the square root of the discriminant will be a real number; otherwise it will be a complex number. If , ''b'', and ''c'' are real numbers then
# If ''b''² − 4''ac'' > 0 then we have two distinct real roots/solutions to the equation .
# If ''b''² − 4''ac'' = 0 then we have one repeated real solution.
# If ''b''² − 4''ac'' < 0 then we have two distinct complex solutions, which are complex conjugates of each other.

Equivalent formulations

The quadratic formula may also be written as

:$x = -\frac \pm \sqrt \ ,$

which may be simplified to

:$x = -\frac \pm \sqrt \ .$

This version of the formula makes it easy to find the roots when using a calculator.

When ''b'' is an even integer, it is usually easier to use the reduced formula

$x = \frac$

In the case when the discriminant $b^2 - 4ac$ is negative, complex roots are involved. The quadratic formula can be written as:

:$x = -\frac \pm i\sqrt \ .$

Muller's method

A lesser known quadratic formula, which is used in Muller's method and which can be found from Vieta's formulas, provides (assuming ) the same roots via the equation:

:$x=\frac = \frac .$

Formulations based on alternative parametrizations

The standard parametrization of the quadratic equation is
:$ax^2+bx+c=0\ .$

Some sources, particularly older ones, use alternative parameterizations of the quadratic equation such as
: $ax^2 - 2b_1 x + c = 0$, where $b_1 = -b/2$,
or
: $ax^2 + 2b_2 x + c = 0$, where $b_2 = b/2$.

These alternative parametrizations result in slightly different forms for the solution, but which are otherwise equivalent to the standard parametrization.

Derivations of the formula

Many different methods to derive the quadratic formula are available in the literature. The standard one is a simple application of the completing the square technique. Alternative methods are sometimes simpler than completing the square, and may offer interesting insight into other areas of mathematics.

By using the 'completing the square' technique

Standard method

Divide the quadratic equation by $a$, which is allowed because $a$ is non-zero:

:$x^2 + \frac x + \frac=0\ .$

Subtract from both sides of the equation, yielding:
:$x^2 + \frac x= -\frac\ \ .$

The quadratic equation is now in a form to which the method of completing the square is applicable. In fact, by adding a constant to both sides of the equation such that the left hand side becomes a complete square, the quadratic equation becomes:

:$x^2+\fracx+\left( \frac \right)^2 =-\frac+\left( \frac \right)^2\ ,$

which produces:

:$\left(x+\frac\right)^2=-\frac+\frac\ \ .$

Accordingly, after rearranging the terms on the right hand side to have a common denominator, we obtain:

:$\left(x+\frac\right)^2=\frac\ \ .$

The square has thus been completed. We can take the square root of both sides, yielding the following equation:

:$x+\frac=\pm\frac\ \ .$

In which case, isolating the $x$ would give the quadratic formula:

:$x=\frac\ \ .$

There are many alternatives of this derivation with minor differences, mostly concerning the manipulation of $a$.

Shorter method

Completing the square can also be accomplished by a sometimes shorter and simpler sequence:

# Multiply each side by ''$4a$'',
# Rearrange.
# Add ''$b^2$'' to both sides to complete the square.
# The left side is the outcome of the polynomial ''$(2ax + b)^2$''.
# Take the square root of both sides.
# Isolate ''$x$''.

In which case, the quadratic formula can also be derived as follows:

:$\begin ax^2+bx+c &= 0 \\ 4 a^2 x^2 + 4abx + 4ac &= 0 \\ 4 a^2 x^2 + 4abx &= -4ac \\ 4 a^2 x^2 + 4abx + b^2 &= b^2 - 4ac \\ (2ax + b)^2 &= b^2 - 4ac \\ 2ax + b &= \pm \sqrt \\\\ 2ax &= -b \pm \sqrt \\ x &= \frac\ \ . \end$

This derivation of the quadratic formula is ancient and was known in India at least as far back as 1025. Compared with the derivation in standard usage, this alternate derivation avoids fractions and squared fractions until the last step and hence does not require a rearrangement after step 3 to obtain a common denominator in the right side.

By substitution

Another technique is solution by substitution. In this technique, we substitute $x = y+m$ into the quadratic to get:

:$a(y+m)^2 + b(y+m) + c =0\ \ .$

Expanding the result and then collecting the powers of $y$ produces:

:$ay^2 + y(2am + b) + \left(am^2+bm+c\right) = 0\ \ .$

We have not yet imposed a second condition on $y$ and $m$, so we now choose $m$ so that the middle term vanishes. That is, $2am + b = 0$ or $\textstyle m = \frac$.

:$ay^2 + y(\ \ \ 0 \ \ ) + \left(am^2+bm+c\right) = 0\ \ .$

:$ay^2 + \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \left(am^2+bm+c\right) = 0\ \ .$

Subtracting the constant term from both sides of the equation (to move it to the right hand side) and then dividing by $a$ gives:

:$y^2=\frac\ \ .$

Substituting for $m$ gives:

:$y^2=\dfrac=\dfrac\ \ .$

Therefore,

:$y=\pm\frac$

By re-expressing $y$ in terms of $x$ using the formula $\textstyle x = y + m = y - \frac$ , the usual quadratic formula can then be obtained:

:$x = \frac\ \ .$

By using algebraic identities

The following method was used by many historical mathematicians:

Let the roots of the standard quadratic equation be and . The derivation starts by recalling the identity:

:$(r_1 - r_2)^2 = (r_1 + r_2)^2 - 4r_1r_2\ \ .$

Taking the square root on both sides, we get:

:$r_1 - r_2 = \pm\sqrt\ \ .$

Since the coefficient , we can divide the standard equation by to obtain a quadratic polynomial having the same roots. Namely,

:$x^2 + \fracx + \frac = (x - r_1)(x-r_2) = x^2 - (r_1 + r_2)x + r_1 r_2\ \ .$

From this we can see that the sum of the roots of the standard quadratic equation is given by , and the product of those roots is given by .

Hence the identity can be rewritten as:

:$r_1 - r_2 = \pm\sqrt = \pm\sqrt = \pm\frac\ \ .$

Now,

:$r_1 = \frac = \frac = \frac\ \ .$

Since , if we take

:$r_1 = \frac$

then we obtain

:$r_2 = \frac\ \ ;$

and if we instead take

:$r_1 = \frac$

then we calculate that

:$r_2 = \frac\ \ .$

Combining these results by using the standard shorthand ±, we have that the solutions of the quadratic equation are given by:

:$x = \frac\ \ .$

By Lagrange resolvents

An alternative way of deriving the quadratic formula is via the method of Lagrange resolvents,Clark, A. (1984). ''Elements of abstract algebra''. Courier Corporation. p. 146. which is an early part of Galois theory.
§6.2, p. 134
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This method can be generalized to give the roots of cubic polynomials and quartic polynomials, and leads to Galois theory, which allows one to understand the solution of algebraic equations of any degree in terms of the symmetry group of their roots, the Galois group.

This approach focuses on the ''roots'' more than on rearranging the original equation. Given a monic quadratic polynomial

:$x^2+px+q\ \ ,$

assume that it factors as

:$x^2+px+q=(x-\alpha)(x-\beta)\ \ ,$

Expanding yields

:$x^2+px+q=x^2-(\alpha+\beta)x+\alpha \beta\ \ ,$

where and .

Since the order of multiplication does not matter, one can switch and and the values of and will not change: one can say that and are symmetric polynomials in and . In fact, they are the elementary symmetric polynomials – any symmetric polynomial in and can be expressed in terms of and . The Galois theory approach to analyzing and solving polynomials is: given the coefficients of a polynomial, which are symmetric functions in the roots, can one "break the symmetry" and recover the roots? Thus solving a polynomial of degree is related to the ways of rearranging (" permuting") terms, which is called the symmetric group on letters, and denoted . For the quadratic polynomial, the only ways to rearrange two terms is to leave them be or to swap them ("transpose" them), and thus solving a quadratic polynomial is simple.

To find the roots and , consider their sum and difference:

:$\begin r_1 &= \alpha + \beta\\ r_2 &= \alpha - \beta\ \ . \end$

These are called the ''Lagrange resolvents'' of the polynomial; notice that one of these depends on the order of the roots, which is the key point. One can recover the roots from the resolvents by inverting the above equations:

:$\begin \alpha &= \textstyle\left(r_1+r_2\right)\\ \beta &= \textstyle\left(r_1-r_2\right)\ \ . \end$

Thus, solving for the resolvents gives the original roots.

Now is a symmetric function in and , so it can be expressed in terms of and , and in fact as noted above. But is not symmetric, since switching and yields (formally, this is termed a group action of the symmetric group of the roots). Since is not symmetric, it cannot be expressed in terms of the coefficients and , as these are symmetric in the roots and thus so is any polynomial expression involving them. Changing the order of the roots only changes by a factor of −1, and thus the square is symmetric in the roots, and thus expressible in terms of and . Using the equation

:$(\alpha - \beta)^2 = (\alpha + \beta)^2 - 4\alpha\beta\ \$

yields

:$r_2^2 = p^2 - 4q\ \$

and thus

:$r_2 = \pm \sqrt\ \$

If one takes the positive root, breaking symmetry, one obtains:

:$\begin r_1 &= -p\\ r_2 &= \sqrt \end$
and thus
:$\begin \alpha &= \tfrac12\left(-p+\sqrt\right)\\ \beta &= \tfrac12\left(-p-\sqrt\right)\ \ . \end$
Thus the roots are
:$\textstyle\left(-p \pm \sqrt\right)$
which is the quadratic formula. Substituting yields the usual form for when a quadratic is not monic. The resolvents can be recognized as being the vertex, and is the discriminant (of a monic polynomial).

A similar but more complicated method works for cubic equations, where one has three resolvents and a quadratic equation (the "resolving polynomial") relating and , which one can solve by the quadratic equation, and similarly for a quartic equation (degree 4), whose resolving polynomial is a cubic, which can in turn be solved. The same method for a quintic equation yields a polynomial of degree 24, which does not simplify the problem, and, in fact, solutions to quintic equations in general cannot be expressed using only roots.

Historical development

The earliest methods for solving quadratic equations were geometric. Babylonian cuneiform tablets contain problems reducible to solving quadratic equations. The Egyptian Berlin Papyrus, dating back to the Middle Kingdom (2050 BC to 1650 BC), contains the solution to a two-term quadratic equation.

The Greek mathematician Euclid (circa 300 BC) used geometric methods to solve quadratic equations in Book 2 of his ''Elements'', an influential mathematical treatise. Rules for quadratic equations appear in the Chinese ''The Nine Chapters on the Mathematical Art'' circa 200 BC. In his work ''Arithmetica'', the Greek mathematician Diophantus (circa 250 AD) solved quadratic equations with a method more recognizably algebraic than the geometric algebra of Euclid. His solution gives only one root, even when both roots are positive.

The Indian mathematician Brahmagupta (597–668 AD) explicitly described the quadratic formula in his treatise ''Brāhmasphuṭasiddhānta'' published in 628 AD,Bradley, Michael. ''The Birth of Mathematics: Ancient Times to 1300'', p. 86 (Infobase Publishing 2006). but written in words instead of symbols. His solution of the quadratic equation was as follows: "To the absolute number multiplied by four times the oefficient of thesquare, add the square of the oefficient of themiddle term; the square root of the same, less the oefficient of themiddle term, being divided by twice the oefficient of thesquare is the value."
This is equivalent to:

:$x = \frac\ \ .$

Śrīdharācāryya (870–930 AD), an Indian mathematician also came up with a similar algorithm for solving quadratic equations, though there is no indication that he considered both the roots.
The 9th-century Persian mathematician Muḥammad ibn Mūsā al-Khwārizmī solved quadratic equations algebraically. The quadratic formula covering all cases was first obtained by Simon Stevin in 1594. In 1637 René Descartes published '' La Géométrie'' containing special cases of the quadratic formula in the form we know today.

Significant uses

Geometric significance

In terms of coordinate geometry, a parabola is a curve whose -coordinates are described by a second-degree polynomial, i.e. any equation of the form:

:$y =p(x) = a_2x^2 + a_1x +a_0\ \ ,$

where represents the polynomial of degree 2 and and are constant coefficients whose subscripts correspond to their respective term's degree. The geometrical interpretation of the quadratic formula is that it defines the points on the -axis where the parabola will cross the axis. Additionally, if the quadratic formula was looked at as two terms,

:$x = \frac=-\frac \pm\frac$

the axis of symmetry appears as the line . The other term, , gives the distance the zeros are away from the axis of symmetry, where the plus sign represents the distance to the right, and the minus sign represents the distance to the left.

If this distance term were to decrease to zero, the value of the axis of symmetry would be the value of the only zero, that is, there is only one possible solution to the quadratic equation. Algebraically, this means that , or simply (where the left-hand side is referred to as the ''discriminant''). This is one of three cases, where the discriminant indicates how many zeros the parabola will have. If the discriminant is positive, the distance would be non-zero, and there will be two solutions. However, there is also the case where the discriminant is less than zero, and this indicates the distance will be ''imaginary'' or some multiple of the complex unit , where and the parabola's zeros will be complex numbers. The complex roots will be complex conjugates, where the real part of the complex roots will be the value of the axis of symmetry. There will be no real values of where the parabola crosses the -axis.

Dimensional analysis

If the constants , , and/or are not unitless, then the units of must be equal to the units of , due to the requirement that and agree on their units. Furthermore, by the same logic, the units of must be equal to the units of , which can be verified without solving for . This can be a powerful tool for verifying that a quadratic expression of physical quantities has been set up correctly, prior to solving this.

See also

* Fundamental theorem of algebra
* Vieta's formulas

References

{{Polynomials

Elementary algebra
Equations