The outer angle at the base of an isosceles triangle is 100 degrees. Find the angle at the apex of the triangle.

1. The sum of adjacent angles is 180 °. Since the outer and inner angles at any vertex of the triangle are adjacent, then the inner angle at the base of the triangle is 180 ° – 100 ° = 80 °;

2. In an isosceles triangle, the angles at the base are equal to each other, so both angles at the base are equal to 80 °;

3. The sum of the angles of a triangle is 180 °, so the angle at the apex of this triangle is 180 ° – 80 ° – 80 ° = 20 °.