A triangle’s three angles sums up to 180 degrees. So the first thing you should do is subtract 50 from 180
180 - 50 = 130
Since it’s an isosceles triangle, ∠A and ∠B are of equal size. So we divide the result, 130, by two. This would give us the m∠A, which is 65.
130/2 = 65
