Question

如图,在△ABC中,AB=AC,CD⊥AB于点D,求证∠A=2∠BCD

Answer 1

AB=AC（说明∠B=∠ACB),则∠B=∠ACD+∠BCD

又CD⊥AB

所以∠A+∠ACD=∠B+∠BCD

∠A+∠ACD=∠ACD+∠BCD+∠BCD

∠A= ∠ACD+∠BCD+∠BCD-∠ACD

∠A=2∠BCD