The acute angles of a right triangle are complementary. The base angles of an isosceles triangle are congruent. If x is the measure of the base angles, this requires ...
x = 90-x . . . . . the base angle is congruent to the other base angle
2x = 90 . . . . . add x
x = 90/2 = 45 . . . . divide by 2
The measure of a base angle of an isosceles right triangle is 45°.
