∠BOC=90-1/2∠A
∠OBC=1/2∠A+1/2∠ACB,
∠OCB=1/2∠A+1/2∠ABC,
∠OBC+∠OCB+∠BOC=180
1/2∠A+1/2∠ACB+1/2∠A+1/2∠ABC+∠BOC=180