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:
