The sum of all the angles of an isosceles triangle is 180 degrees.

The sum of all the angles of an isosceles triangle is 180 degrees. Find the angles at the base of the triangle if the third angle is 90 degrees.

Let us take the value of the angle at the base as x, since the rectangle is isosceles, then from its properties it follows that the other angle at the base is also equal to x.
Since the sum of the angles of the triangle is 180 degrees, we get the equation:
x + x + 90 = 180
2 * x = 180-90 = 90
x = 45.