Illustrative Rein M. Mathematics Lesson 9: Conditions for Triangle Similarity Cool Down: Any Four Angles? Priya noticed in the last activity that between the 2 triangles, you only need to know 4 angles to show that they are similar. She wondered which fourth angle would work to prove that triangle RST is similar to triangle EFG. In triangle RST: In triangle EFG: . Angle R is 90 . Angle E is 90 . Angle S is 25 . Angle F is y . Angle T is x . Angle G is zo Draw a sketch of the triangles. Then pick one angle measurement that would prove the triangles are similar. Explain to Priya why knowing that angle would be enough