The degree measure of the outer angle A of an isosceles triangle ABC (AB = BC)

The degree measure of the outer angle A of an isosceles triangle ABC (AB = BC) is 125 degrees. Find the degree measure of the inner angle B.

Since the outer corner at the vertex A and the inner corner A are adjacent angles, then the angle A is 180 ° – 125 ° = 55 °.

The ABC triangle is isosceles, in an isosceles triangle the angles at the base are equal, so the angle C is equal to the angle A = 55 °.

The sum of the angles in a triangle is 180 °, hence the angle B = 180 ° – (55 ° + 55 °) = 180 ° – 110 ° = 70 °.

Answer: The degree measure of angle B is 70 °.