In triangle ABC, angle C = 20 °, AC = BC find angle A.

Since in this triangle BC = AC, this triangle is isosceles with base AB.

According to the properties of triangles, in an isosceles triangle, the angles at the base are equal:

∠А = ∠В.

Since the sum of all the angles of the triangle is 180º, then:

∠А = ∠В = (180º – ∠С) / 2;

∠А = ∠В = (180º – 20º) / 2 = 160º / 2 = 80º.

Answer: the degree measure of the angle ∠A is equal to 80º.