⑩天掌 Teaching Plan on Advanced Mathematics 关于数量积的说明: ()ad=ldl 证:日=0,∴a·a= alla cos=l (2)a·b=0←→a⊥b,(≠0,b≠0) 证(→)∵·b=0,|a≠0,|b≠0, C0S6=0,0=T (÷)∵⊥b,∴0=,c0s6=0, d·b=4b|c0s8=0.Tianjin Polytechnic University Teaching Plan on Advanced Mathematics 关于数量积的说明： (2) a  b = 0    a b,   ⊥ () a  b = 0,    | a | 0,  | b | 0,  a b.    ⊥ (1) | | . 2 a a a     = () a b,    ⊥ cos = 0,  = 0, | || | cos | | . 2 a a a a a      证   =  = 证 cos = 0,  = , 2  , 2   = ( 0, 0).     a  b  a  b =| a || b | cos = 0.    