应用:(i)|ab|=arcs-ab )同 (iii) albea.b=0 例1、习题4,1选择题(1)(2)(3) 2填空题(3)(4)(5) 例2、设同=5问=2|a5-,则-3 解:2-32=(2a-5)(a-5)=42-2a:6+92=76 (2)向量积 =ax b即axba,axb⊥b, 右手定则 即×b),a=0, b=0 性质P155注意a×b=-b×a b应用:(i) a b a b a b arccos − = (ii) 2 a a a a = = (iii) a⊥b a b = 0 例 1、习题 4,1 选择题(1)(2)(3) 2 填空题(3)(4)(5) 例 2、设 , 则 2a 3b 2 19 3 π a 5, b 2, a b = − = = = 解: 2a 3b (2a 3b) (2a 3b) 4 a 2a b 9b 76 2 2 2 − = − − = − + = ∴ 2a − 3b = 2 19 (2)向量积 a b c = ( ) c = a b = a b sin a, b c a, c b a b a, a b b, ⊥ ⊥ 即 ⊥ ⊥ 右手定则 即 (a b) a = 0, (a b) b = 0 性质 P155 注意 a b b a = − x y z x y z b b b a a a i j k a b =