过A作AD⊥BC于D
BP=AC,
∠PMB=∠ADC
∠EBC+∠BCE=∠DAC+∠ACD=90°
∠EBC=∠DAC
BP=AC
△ADC≌△BDP
PM=DC
同理∠BAD+∠ABC=∠QCN+∠ABC=90°
∠BAD=∠QCN,直角,CQ=AB
△ADB≌△CNQ
QN=BD
BD+DC=BC
PM+QN=BC
若△ABC中∠A为钝角,同样方法得PM+QN=BC
若△ABC中∠C为钝角,同样方法得QN-PM=BC
若△ABC中∠B为钝角,同样方法得PM-QN=BC