证明:因P为AM中点,Q为BN中点 PQ=PM+MN+QN
所以AP=PM QB=QN
所以2PQ=2PM+2QN+2MN
又因为AB=AP+PM+MN+NQ+QB
所以MN+AB=AP+PM+MN+NQ+QB=MN
=2PM+2QN+2MN
=2PQ
证明:因P为AM中点,Q为BN中点 PQ=PM+MN+QN
所以AP=PM QB=QN
所以2PQ=2PM+2QN+2MN
又因为AB=AP+PM+MN+NQ+QB
所以MN+AB=AP+PM+MN+NQ+QB=MN
=2PM+2QN+2MN
=2PQ