A triangle is shown with a leg extending past the top vertex. The vertices are labeled ABC. Angle y is located inside the triangle at vertex B. Angle z is located inside the triangle at vertex C. Angle x is located outside the triangle between beside vertex A and the extended leg. Angle r is located inside the triangle at vertex A. x + z = y
180 degrees − x = r
x + y + z = 180 degrees
x + y + z = 90 degrees