

Geometry Problem 1120: Isosceles Right
Triangle, 120 Degree, Angle, Equilateral, Metric
Relations. Level: High School, SAT Prep,
Honors Geometry, College, Math Education
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The figure below shows an isosceles right triangle ABC.
D is a point so that angle ADC = 120 degree. E is on CD extended
so that angle BEC = 120 degree. BE extended meets AD at F. If AD
= d, CE = e, and BF = f, prove that (1) triangle DEF is
equilateral; (2)
.

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