数值分析--经过三点的二次函数

经过三点(x1,y1),(x2,y2),(x3,y3)的二次函数用c++语言表示如下：

double  quad(double x1,double y1,double x2,double y2,double x3,double y3,double x)
{
double m1 = (x-x2)*(x-x3)*y1/((x1-x2)*(x1-x3)),
m2 = (x-x1)*(x-x3)*y2/((x2-x1)*(x2-x3)),
m3 = (x-x1)*(x-x2)*y3/((x3-x1)*(x3-x2));

return (m1+m2+m3);

}