Angle: △AEC congruence △ADB, ∠AD=∠DE=∠AE (congruence)
(2) Prove: ∫△ABC congruence△△ ade
∴∠D=∠B (the corresponding angles of congruent triangles are equal).
∠B=20
∴BC=DE (the corresponding sides of congruent triangles are equal)
DE=5
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