Given: AB=AC, DE ǁ BC

Prove: AD=AE

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Substitution
Parallel lines form equal corresponding angles
∠D=∠E
∠B = ∠C
∠D = ∠B and ∠E = ∠C
If two angles of a triangle are equal, the sides opposite them are equal
AB=AC, DE ǁ BC
Given
AD=AE
If two sides of a triangle are equal, the angles opposite them are equal