∠1 + ∠2 + ∠3 = ∠DAE

Substitution

Definition

∠1 + ∠2 + ∠3 = 180

Substitution

Angle addition theorem

∠DAE is a straight angle

∠DAC + ∠3 = ∠DAE

Angle addition theorem

∠1 + ∠2 = ∠DAC

∠DAE = 180

Given