∠C的外角=∠A+∠B,△DBC中,∠D+∠DCB+∠ABC=180,
即:∠D+1/2(∠A+∠B)+∠ACB+1/2∠B=180,
又△ABC中,∠A+∠B+∠ACB=180,两式想减得::∠D=1/2∠A