∠B'PB与∠BPD对称与直线BP,
∠B'PB=∠BPD,
∠C'PA与∠APC对称与直线AP,
∠C'PA=∠APC,
2(∠B'PB+∠C'PA)=180度,
∠B'PB+∠C'PA)=90度,
∠APB=90度.