1)两向量的数量积是个数量 ,而不是向量 .它的值为两向量的模与其夹角余弦的乘积 ,其符号是由夹角θ(0≤θ≤π)决定的 .θ为锐角 ,数量积为π ;θ为钝角 ,数量积为负 ;θ为直角 ,数量积为零 ;θ =0 ,a·b =|a| |b| ,a·a =|a| 2 ,(a±b) 2 =a2 ± 2a·b +b2 =|a| 2 ±2 |a| |b 1) The scalar product of two vectors is a quantity, not a vector. Its value is the product of the two-vector mode and the cosine of its included angle. Its sign is determined by the angle θ (0≤θ≤π). θ is an acute angle. The quantity product is π; θ is an obtuse angle, the quantity product is negative; θ is a right angle, the quantity product is zero; θ =0,a·b =|a| |b|, a·a =|a| 2 ,(a± b) 2 =a2 ± 2a·b +b2 =|a| 2 ±2 |a| |b