⑴.β=180°-(∠B+∠C)/2=90°+α/2.
⑵.∠B/2+∠C+(180°-∠C)/2+β=180°.α=180°-∠B-∠C.
算得β=α/2.
⑶.β=180°-[(180°-∠B)/2+(180°-∠C)/2]=90°-α/2.