CSSTP is an acronym that represents a simple but useful truth: In similar triangles, the Corresponding Sides of Similar Triangles are Proportional. CSSTP proofs often involve an odd step at the end where you have to prove that one product of sides equals another product of sides.
You'll see what this means in the following problem:
Here's the proof diagram.
You can often use a proportion to prove that two products are equal; therefore, if you're asked to prove that a product equals another product
the proof probably involves a proportion related to similar triangles. So look for similar triangles that contain the four segments in the prove statement. You can then set up a proportion using those four segments and finally cross-multiply to arrive at the desired product.
Here's the formal proof:
