过P作PM⊥AB,垂足M,延长MP,交CD于N
因为平行四边形ABCD
因为AB‖CD,AB=CD
故：PN⊥CD,MN⊥CD,MN⊥AB
因为平行线间的垂线段相等
故：MN是平行四边形ABCD以AB或CD为底的高
故：S△PAB+S△PCD=1/2•AB•PM+1/2•CD•PN
=1/2•AB•PM+1/2•AB•PN
=1/2•AB•(PM+PN)
=1/2•AB•MN
=1/2•S平行四边形ABCD
=50