Given: δdfe is isosceles with base fe; fb ≅ ec. prove: δdfb ≅ δdec complete the missing parts of the paragraph proof. we know that triangle dfe is isosceles with base fe and that segment fb is congruent to segment ec because . segment df is congruent to segment by the definition of isosceles triangle. since these segments are congruent, the base angles, angles , are congruent by the isosceles triangle theorem. therefore, triangles are congruent by sas