关于数量积的说明: I·=a 证∵6=0,∴l·a=l‖lic0sb=l (2)a·b=0←→a⊥b 证(→):a·b=0,|a≠0,|b≠0, c0S6=0,0=兀 d⊥b 2 T (÷):d⊥b,6 cos=0 2 d·b=bc0s=0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   =