1平面向量数量积“性质1”[1]的解读设a,b都是非零向量,e是与b方向相同的单位向量e=|bb|,θ是a与e的夹角.则(1)e·a=a·e=|a|cosθ|bb|·a=a·|bb|=|a|cosθ|a·b|b=|a|cosθ(2)|a·b|b=|a|cosθ都表示a在b方向上的射影(课本上称投影.)(3)a在b方向上的射影(投影)的长度d=|a·b| 1 Interpretation of “Nature 1” [1] Let a and b be non-zero vectors, e is the unit vector e = | bb | which is the same as the direction of b, and θ is the angle between a and e. ) a = a · e = | a | cos θ | bb | · a = a · bb | = | a | cos θ | a · b | b = | a | cos θ (2) | a · b | b = | a | cos θ both represent the projection of a in the direction of b (textbook projection.) (3) a The length of the projection (projection) in the direction of b d = | a · b |