Hello, MATNTRNG!

I have my own proof of this . . .Code:A o * * r * @ * * * O * o * * 2@* r * @ * r * * * * C o * * * o B

Right triangle can be inscribed in a semicircle.

. . is the center of the circle

. . and are radii of the circle.

Hence, is isosceles and

is an exterior angle of .

. . Hence: .

Therefore: .