# Isosceles and equilateral triangles

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##### Examples
###### Lessons
1. Find x.

2. In $\triangle DEF$, $\overline {EY}$ is a perpendicular line to $\overline {DF}$ and $Y$ is the midpoint of $\overline {DF}$. Why is $\triangle DEF$ an isosceles triangle? Show your reasoning.