过程有点省略
但愿您能看懂.
∠EAD=90°-∠AED
=90°-∠B-1/2∠A
=90°-∠B-1/2(180°-∠B-∠C)
=90°-∠B-90°+1/2∠B+1/2∠C
=1/2(∠C-∠B)