# 166 geometry/circles.and.triangles.p

## Description

This article is from the Puzzles FAQ,
by Chris Cole chris@questrel.questrel.com and Matthew Daly
mwdaly@pobox.com with numerous contributions by others.

# 166 geometry/circles.and.triangles.p

Find the radius of the inscribed and circumscribed circles for a triangle.

geometry/circles.and.triangles.s

Let a, b, and c be the sides of the triangle. Let s be the

semiperimeter, i.e. s = (a + b + c) / 2. Let A be the area

of the triangle, and let x be the radius of the incircle.

Divide the triangle into three smaller triangles by drawing

a line segment from each vertex to the incenter. The areas

of the smaller triangles are ax/2, bx/2, and cx/2. Thus,

A = ax/2 + bx/2 + cx/2, or A = sx.

We use Heron's formula, which is A = sqrt(s(s-a)(s-b)(s-c)).

This gives us x = sqrt((s-a)(s-b)(s-c)/s).

The radius of the circumscribed circle is given by R = abc/4A.

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