Question

Write an indirect proof proving the two non-right angles of a right triangle measure less than 90°.

Answer 1

First, you must realize that the sum of all angles of any triangles is 180 degrees

Answer 2

So if the 2 nonright angles of a right triangle are a and b, a+b+90=180
a+b=90

Answer 3

If we assume that there is another right angle, than the other angle is 0, which is impossible.
So, that is how it was proven.