Isosceles and equilateral triangles

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Examples
Lessons
  1. Find x.

    1. equilateral triangles and angles inside the triangles

    2. determining the side length of equilateral triangles

    3. Isosceles and equilateral triangles

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