''The Secrets of Triangles: A Mathematical Journey'' is a popular mathematics book on the

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 ...

triangle A triangle is a polygon with three edges and three vertices. It is one of the basic shapes in geometry. A triangle with vertices ''A'', ''B'', and ''C'' is denoted \triangle ABC. In Euclidean geometry, any three points, when non- colline ...

s. It was written by Alfred S. Posamentier and , and published in 2012 by Prometheus Books.

Topics

The book consists of ten chapters, with the first six concentrating on

triangle center In geometry, a triangle center (or triangle centre) is a point in the plane that is in some sense a center of a triangle akin to the centers of squares and circles, that is, a point that is in the middle of the figure by some measure. For exampl ...

s while the final four cover more diverse topics including the

area Area is the quantity that expresses the extent of a region on the plane or on a curved surface. The area of a plane region or ''plane area'' refers to the area of a shape or planar lamina, while '' surface area'' refers to the area of an ope ...

of triangles,

inequalities Inequality may refer to: Economics * Attention inequality, unequal distribution of attention across users, groups of people, issues in etc. in attention economy * Economic inequality, difference in economic well-being between population groups * ...

involving triangles,

straightedge and compass construction In geometry, straightedge-and-compass construction – also known as ruler-and-compass construction, Euclidean construction, or classical construction – is the construction of lengths, angles, and other geometric figures using only an ideali ...

s, and fractals. Beyond the classical triangle centers (the

circumcenter In geometry, the circumscribed circle or circumcircle of a polygon is a circle that passes through all the vertices of the polygon. The center of this circle is called the circumcenter and its radius is called the circumradius. Not every polyg ...

incenter In geometry, the incenter of a triangle is a triangle center, a point defined for any triangle in a way that is independent of the triangle's placement or scale. The incenter may be equivalently defined as the point where the internal angle bis ...

orthocenter In geometry, an altitude of a triangle is a line segment through a vertex and perpendicular to (i.e., forming a right angle with) a line containing the base (the side opposite the vertex). This line containing the opposite side is called the '' ...

, and

centroid In mathematics and physics, the centroid, also known as geometric center or center of figure, of a plane figure or solid figure is the arithmetic mean position of all the points in the surface of the figure. The same definition extends to any ...

) the book covers other centers including the Brocard points,

Fermat point In Euclidean geometry, the Fermat point of a triangle, also called the Torricelli point or Fermat–Torricelli point, is a point such that the sum of the three distances from each of the three vertices of the triangle to the point is the smallest ...

Gergonne point In geometry, the incircle or inscribed circle of a triangle is the largest circle that can be contained in the triangle; it touches (is tangent to) the three sides. The center of the incircle is a triangle center called the triangle's incenter. ...

, and other geometric objects associated with triangle centers such as the

Euler line In geometry, the Euler line, named after Leonhard Euler (), is a line determined from any triangle that is not equilateral. It is a central line of the triangle, and it passes through several important points determined from the triangle, includ ...

Simson line In geometry, given a triangle and a point on its circumcircle, the three closest points to on lines , , and are collinear. The line through these points is the Simson line of , named for Robert Simson. The concept was first published, howeve ...

, and

nine-point circle In geometry, the nine-point circle is a circle that can be constructed for any given triangle. It is so named because it passes through nine significant concyclic points defined from the triangle. These nine points are: * The midpoint of ea ...

. The chapter on areas includes both

trigonometric Trigonometry () is a branch of mathematics that studies relationships between side lengths and angles of triangles. The field emerged in the Hellenistic world during the 3rd century BC from applications of geometry to astronomical studies. ...

formulas and

Heron's formula In geometry, Heron's formula (or Hero's formula) gives the area of a triangle in terms of the three side lengths , , . If s = \tfrac12(a + b + c) is the semiperimeter of the triangle, the area is, :A = \sqrt. It is named after first-century ...

for computing the area of a triangle from its side lengths, and the chapter on inequalities includes the

Erdős–Mordell inequality In Euclidean geometry, the Erdős–Mordell inequality states that for any triangle ''ABC'' and point ''P'' inside ''ABC'', the sum of the distances from ''P'' to the sides is less than or equal to half of the sum of the distances from ''P'' to the ...

on sums of distances from the sides of a triangle and

Weitzenböck's inequality In mathematics, Weitzenböck's inequality, named after Roland Weitzenböck, states that for a triangle of side lengths a, b, c, and area \Delta, the following inequality holds: : a^2 + b^2 + c^2 \geq 4\sqrt\, \Delta. Equality occurs if and on ...

relating the area of a triangle to that of squares on its sides. Under constructions, the book considers 95 different triples of elements from which a triangle's shape may be determined (taken from its side lengths, angles, medians, heights, or angle bisectors) and describes how to find a triangle with each combination for which this is possible. Triangle-related fractals in the final chapter include the Sierpiński triangle and

Koch snowflake The Koch snowflake (also known as the Koch curve, Koch star, or Koch island) is a fractal curve and one of the earliest fractals to have been described. It is based on the Koch curve, which appeared in a 1904 paper titled "On a Continuous Curv ...

Audience and reception

Reviewer Alasdair McAndrew criticizes the book as being too "breathless" in its praise of the geometry it discusses and too superficial to be of interest to professional mathematicians, and Patricia Baggett writes that it little of its content would be of use in high school mathematics education. However, Baggett suggests that it may be usable as a reference work, and similarly Robert Dawson suggests using its chapter on inequalities in this way. The book is written at a level suitable for high school students and interested amateurs, and McAndrew recommends the book to them. Both Baggett and Gerry Leversha find the chapter on fractals (written by Robert A. Chaffer) to be the weakest part of the book, and Joop van der Vaart calls this chapter interesting but not a good fit for the rest of the book. Leversha calls the chapter on area "a bit of a mish-mash". Otherwise, Baggett evaluates the book as "well written and well illustrated", although lacking a glossary. Robert Dawson calls the book "very readable", and recommends it to any mathematics library.

References

{{DEFAULTSORT:Secrets of Triangles Popular mathematics books Triangle geometry 2012 non-fiction books Prometheus Books books

Topics

Audience and reception

See also

References