Geometry For Dummies, 3rd Edition
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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:

geometry-CSSTP

Here's the proof diagram.

geometry-csstp-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

geometry-prove-statement

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:

geometry-formal

About This Article

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About the book author:

Mark Ryan is the founder and owner of The Math Center in the Chicago area, where he provides tutoring in all math subjects as well as test preparation. Mark is the author of Calculus For Dummies, Calculus Workbook For Dummies, and Geometry Workbook For Dummies.

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