Question: how to find the measure of a base angle of an isosceles triangle?
Answer: So the two base angles must add up to 180-40, or 140°. Since the two base angles are congruent (same measure), they are each 70°. If we are given a base angle of say 45°, we know the base angles are congruent (same measure) and the interior angles of any triangle always add to 180°.