由平面向量的数量积公式:a·b=|a|·|b|cosθ(其中θ为非零向量a与b的夹角),我们容易得到下面的结论:-|a|·|b|≤a·b≤|a|·|b|.当a与b共线且方向相同时,右边的不等式取等号;当a与b共线且方向相反时,左边的不等式取等号.这个结论在证明不等式和求函数的最值时常常用到,使用的? From the quantity product formula of a plane vector: a·b=|a|·|b|cosθ (where θ is the angle between non-zero vectors a and b), we easily get the following conclusion: -|a|·|b| ≤a·b≤|a|·|b|. When a and b are collinear and the directions are the same, the right inequality takes the equal sign; when a and b are collinear and the directions are opposite, the left inequality takes the equal sign. Conclusion is often used in the proof of inequality and find the function of the value of the use?