So, we are given a triangle ABC isosceles, AC = BC. It is also known that the angle A is equal to 80 °. In order to find the angle C, we will use the theorem on the sum of the angles of a triangle and the properties of the angles at the base of an isosceles triangle.
So angle C is the angle at the apex of an isosceles triangle, and angles A and B are angles at the base.
The angles at the base of an isosceles triangle are equal to each other, that is, angle A is equal to angle B and equal to 80 °.
Looking for corner C:
180 ° – (80 ° + 80 °) = 180 ° – 160 ° = 20 °
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