Foundations of Similar Triangles

Path to the Proof

Thirteen short steps from your very first right angle to the crossing-diagonals problem below.

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Original Target Problem

The problem we are training for

Rectangle ABCD has a right angle at B. AB = DC = 20. Point E lies on DC so that DE = 3·EC. Diagonal AC and segment BE cross at point F. Find AF : FC.

Scroll down — by the time you reach Unit 10, you will be able to solve this yourself using nothing more than a right angle, a pair of parallel sides, and one similarity rule.

📓 Wrong-Answer Notebook

Every question you missed or skipped, gathered here for a second look.