平面向量的数量积是一个重点、难点 .学生对平面向量的数量积及其性质的应用 ,往往感到困难、或无从入手 .本文从以下几个方面讲解它的性质及应用 .两个非零向量 a和 b,它们的夹角为θ,把数量 | a| b| cosθ叫做 a和 b的数量积 (或内积 ) ,即 a . b =| a| | b| cosθ.1　数量积 The quantity product of the plane vector is a key and difficult point. The student’s application of the quantity product and its nature of the plane vector is often difficult or impossible to start. This article explains its nature and application from the following aspects. Two non-zero vectors a and b, their angles are θ, and the number |a| b| cos θ is called the quantity product (or inner product) of a and b, ie, a . b =| a| | b| cosθ.1