∠ADE=∠B+∠BAD
=∠B+1/2∠BAC
=∠B+1/2(180-∠B-∠C)
=90+1/2(∠B-∠C)
∠DAE=90-∠ADE
=90-90-1/2(∠B-∠C)
=1/2(∠C-∠B)