Maths problem: how to prove, thanks?

∠EAD = ∠FAD
∠AED = ∠AFD
By AAS, △AED 全等 △AED
∴AE = AF,
設 AD線段 與 EF線段 交於 G,
則 by SAS, △AEG 全等 △AFG
∴ ∠AGE = ∠AGF = 90°, 且 EG = FG.