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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