解:由比蚂含例关系得AE/AD=PN/BC(80-ED)/80=PN/120 ED=PQPN=-2/3*PQ+120矩形PQMN的面积=PQ*PN=-2/闷行笑3*PQ*PQ+120*PN=-2/3(PQ-40)*(PQ-40)+1600即当PQ=40cm时 矩带埋形PQMN的面积最大=1600cm*cmpn=-2/3*80+120=280/3cm
长60,宽40