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On page 261 (question 11c) of the ETS OG, they ask for the length of the diagonal of a parallelogram. I don't see a formula for calculating the diagonal in the OG. It also doesn't seem to be in the MP Geometry guide. Did I miss it?
There is no formula that doesn't involve trigonometry. In the question you're talking about, they give you extra information that allows you to simply solve for the diagonal of the right triangle. They'd have to do that on the real test, too.