z = x^2+y^2, 记 F = x^2+y^2 - z, 则Fx = 2x, Fy = 2y, Fz = -1, 在点 P(-1,-1,2) 处 Fx = -2, Fy = -2, Fz = -1法线方程 (x+1)/(-2) = (y+1)/(-2) = (z-2)/(-1)即 (x+1)/2 = (y+1)/2 = (z-2)/1
