1.a·b的几何意义是a·b等于a的长度与b在a的方向上的投影的乘积. 2.计算数量积,找角是关键,可将两个向量平移到同一起点再找.例如:在图1所示的正三角形ABC中,求<■>.可将(■)平移到(■),则∠DCB= 120°就是所求. 3.涉及长度或两点间距离的问题,以及两直线夹角或垂直问题, The geometrical meaning of 1.a·b is that a·b is equal to the product of the length of a and the projection of b in the direction of a. 2. Calculating the quantity product, the finding angle is the key, the two vectors can be translated to the same starting point to find For example: In the regular triangle ABC shown in Fig. 1, find <■>. You can translate (■) to (■), then ∠ DCB = 120° is the required. 3. Involving length or distance between two points Problems, and two straight-line or vertical problems,