设F(x,y,z) = z-x^2-y^2+1
那么F'(x) = -2x
F'(y) = -2y
F'(z) = 1
所以在点(2,1,4)处的法向量为(-4,-2,1)或(4,2,-1)
法线方程为(x-2)/4=(y-1)/2=4-z
切平面方程为4(x-2)+2(y-1)-z+4=0