关于数量积的说明: (1)a·a=l 证∵6=0,∴l·=l‖lic0sb=l (2)a.b=0←→d⊥b 证(→):·b=0,|l|≠0,|b≠0, c0s6=0,θ=π,∴db 2 (÷)∵a⊥b,6 cos=0 2 n·b=l‖b|cos6=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   =