Given a triangle ABC, AB = AC. The outside angle at the vertex A is 110. Find the angle B.

We are given an isosceles triangle, the outer angle at the vertex A is known, it is equal to 110 °. In order to find the angle at the base of an isosceles triangle, we must remember what the sum of adjacent angles is equal to and thus calculate the degree measure of the angle of the triangle at the vertex.

The sum of adjacent angles is 180 °.

180 ° – 110 ° = 70 ° angle at the apex of an isosceles triangle.

We also know that the angles at the base of an isosceles triangle are equal to each other. And the sum of the angles of a triangle is 180 °.

180 ° – 70 ° = 110 °.

110 °: 2 = 55 ° angle at the base of an isosceles triangle.