ΔMQC全等于ΔMNB(边角边)
->∠NBM=∠QCM,故∠NBP=∠NBM+∠CBA=∠ACB+∠CBA=90°,即NB⊥AB
PQ^2=PM^2+MQ^2=PM^2+MN^2=PN^2=BP^2+BN^2=BP^2+Q^2.