In an isosceles triangle, the base angle is 30 degrees, what is the apex angle?

Given:

triangle ABC – isosceles,

АС – base,

angle A = 30 °.

Find: Angle B

Solution: Consider an isosceles triangle ABC. In an isosceles triangle, the angles at the base are equal, that is, the angle A = angle C = 30 °. We know that the sum of the degree measures of the triangle is 180 °, then the angle A + angle B + angle C = 180 °.

We get:

angle B = 180 ° – angle A – angle C;

angle B = 180 ° – 30 ° – 30 °;

angle B = 120 °.

Answer: 120 °.