根据三个点的坐标计算其夹角

求给定三个点的夹角，其实是求 $< \vec{A}, \vec{B} >$ 两个向量之间的夹角 $α$

cos α = A \times B/ ∥ A ∥ ∥ B ∥

源代码（Java）如下：

private double getAngleByThreeP(double[] pointx, double[] pointy) {
        double a_b_x = pointx[0] - pointx[1];
        double a_b_y = pointy[0] - pointy[1];
        double c_b_x = pointx[2] - pointx[1];
        double c_b_y = pointy[2] - pointy[1];
        double ab_mul_cb = a_b_x * c_b_x + a_b_y * c_b_y;
        double dist_ab = Math.sqrt(a_b_x * a_b_x + a_b_y * a_b_y);
        double dist_cd = Math.sqrt(c_b_x * c_b_x + c_b_y * c_b_y);
        double cosValue = ab_mul_cb / (dist_ab * dist_cd);
        return Math.acos(cosValue);
    }

求出来的结果是弧度值（另：角度值=弧度*180/π）

转载自【搬砖程序员】