叉乘公式啊,列个行列式,第一行i,j,k,第二行和第三行分别是叉乘的两项,将M点的坐标代入就ok了,so easy (note: i, j and k represent the corresponding unit vector along the direction of axis x, y and z)
When you don't know how to solve a sophisticated problem, find out its definition and think it over until you can figure out the problem.
追问没学行列式