Similar triangles have equal angle measures. The sum of the angles of any triangle must add up to 180°. Since the angles of ΔDEF are 80° and 40°, then the third angle must be 60°: 80 + 40 = 120; 180 - 120 = 60. Also, the angles of ΔABC are 60° and 40°, which makes the third angle 80°: 60 + 40 = 100; 180 - 100 = 80. Given that ΔDEF and ΔABC both have angles of 40°, 60° and 80°, then they are similar.
