关于数量积的说明: aa=al 证∵=0,∴a,a=a‖lcos=|2 (2)ab=0←→a⊥b 证(→)a,b=0,1a≠=0,1b≠0, 工工工 c0sb=0,=“,∴a⊥b 2 (←)∵a⊥b,∴6= ∴cosb=0. 2 d·b=|bcos6=0. 上页关于数量积的说明： (2) a b = 0    a b .   ⊥ () a  b = 0,    | a | 0,  | b | 0,   cos = 0, a b .    ⊥ (1) | | . 2 a a a     = () a b ,    ⊥  cos = 0, a b =| a ||b | cos = 0.      = 0, | | | | cos | | . 2 a a a a a      证   =  = 证  = , 2  , 2   =