Given the sides of a triangle, what is the length of the angle bisector? Consider the following image of triangle ABC, where the sides opposite points A, B, C have lengths a, b, c. We want to find length d.

First, we can apply the Angle Bisector Theorem to see that , so that and . Plugging these into the equation yields and , so that and .

We can now apply Stewart’s Theorem to see that . Plugging in our expressions for x and y, we see that

.

Therefore, the length of the angle bisector at vertex C is

.

The lengths of the other two angle bisectors can be found analogously and are given by a simple permutation of coordinates.

Here’s another way to compute the length of the angle bisector that might be a bit simpler: Let be , so that the angle bisector separates two angles with measure . The area of triangle ABC is the sum of the areas of the two smaller triangles, which can be expressed as . Simplifying and rearranging, we see that , so the length of the angle bisector is therefore

.

This is simpler than what we had before, though it also involves an angle.

We can use this expression for the length of the angle bisector to show that the sum of lengths of the angle bisectors is less than the perimeter. The length of the angle bisector to angle C is , and the other angle bisectors are similar. Therefore, it is sufficient to show that

.

This inequality actually follows as a simple application of the arithmetic mean – harmonic mean (AM-HM) inequality, which states that

.

Applying this three times yields

,

which is what we wanted to show. Therefore, we've proven that the sum of lengths of angle bisectors in a triangle is less than the perimeter.

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Your formula

when applied to the following figure (based on a sample problem from the

The New York Times’articleU.S. Falls Short in Measure of Future Math Teachers)where

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