If we calculate the projection coordinates of A on the surface BCD, we must first calculate the vectors BC, CD and BD, let the normal vector of the surface BCD be n, and the product of the normal vector and the vectors BC, CD and BD be 0 respectively, and then we can write the point-normal plane equation of BCD, express the straight lines parallel to A and N in a point-to-point way, and then substitute a parametric equation for the straight lines and substitute them into the plane equation.