∵∠DCE=∠D+∠DBE=∠D+1/2 ∠ABE∵∠ACE=∠A+∠ABE∵CD平分∠ACE, BC平分∠ABE,∴∠DCE=∠D+1/2 ∠ABE=1/2(∠A+∠ABE)∴∠D+1/2 ∠ABE=1/2∠A+1/2∠ABE∴∠D=1/2∠A
