1、直四棱柱,则AB⊥平面A1B1C1D1,则：AA1⊥平面ABCD,则：AB⊥AA1,又底面就矩形,则AB⊥AD,则AB⊥平面ADD1A1,则AB⊥A1D
2、取A1D1中点H,证明AMNH是平行四边形即可.