Answer: An isosceles triangle is a triangle with two sides that are congruent (equal in length). If DEF is isosceles with base DF, then DE and EF are congruent and have the same length. This means that the two sides of the triangle are equal in length, and that the angle formed by DE and EF is equal to each other (since they are congruent sides, they must also be congruent angles). The two equal angles are called the base angles of the isosceles triangle.
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