Then because AB=AC

∠ 1+∠2=∠ABC=∠C

∠ 1=∠C-∠2 ①

And BD⊥AC.

∠A+∠ 1=90

∠2+∠C=90

∠2=90 -∠C ②

∠ A = 90-∠ 1, substituted into ①.

∠ A = 90-∠ C+∠ 2, which can be obtained from ②.

∠A=∠2+∠2

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