设∠B=B,∠C=C.
则∠BAD=90°-B,∠CAE=(180°-B-C)/2
则∠AED=∠BAD=∠EAC-∠BAC=(360°-2B)/2+(180°-B-C)/2
-(360-2B-2C)/2=1/2(∠C-∠B).