AB is the base of an isosceles triangle. Apex angle B is 31 degrees. Find other angles of triangle ABC.

Given:
ΔABS – isosceles
AB – the base of the triangle
∠В = 31 °
To find:
∠А -?
∠C -?

Decision:
ABC – isosceles triangle with base AB.
In isosceles triangles, the angles at the base are equal, therefore, ∠A = ∠B = 31 °
The sum of the angles of any triangle is 180 °, ∠A = ∠B = 31 °. Hence:
∠А + ∠В + ∠С = 180 °
∠С = 180 ° -∠А-∠В
∠С = 180 ° -31 ° -31 °
∠С = 118 °

Answer: ∠А = 31 °, ∠С = 118 °.