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$ABCDE$ is a regular pentagon of side length one unit.
The lines $AB$ and $DC$ are extended until they meet at another point, $F$.
Can you show that triangle $BFC$ is congrugent to triangle $DAC$?
Can you find a pair of similar triangles?
Can you use these results to find the length $DA$?
You can read more about the Golden Ratio in this Plus Article.
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