Isosceles Triangle

Free interactive math game: Isosceles Triangle — Find the angle. Part of the Geometry (Angles) category on MathKite.

Example problems

Taken from the generator that powers this exercise. A fresh set appears every time the page is opened, and the printable worksheet holds ten of them.

  1. In an isosceles triangle each base angle is 57°. Find the vertex angle.
  2. In an isosceles triangle each base angle is 68°. Find the vertex angle.
  3. In an isosceles triangle each base angle is 35°. Find the vertex angle.
  4. In an isosceles triangle each base angle is 70°. Find the vertex angle.

About this exercise

Angles in an isosceles triangle.

How to solve it

The two angles opposite the equal sides are equal. From the apex, subtract it from 180 and halve. From a base angle, double it and subtract from 180.

Worked example

An apex of 40 leaves 140, so each base angle is 70. Base angles of 37 leave 180 - 74 = 106 at the apex.

Common mistake

Halving before subtracting, or doubling the wrong angle. Only the two base angles are equal.