yz = ln(x+z) So I'm trying to find the tangent plane to the surface at a particular point (x0,y0,z0). Here's the general formula: Fx(x0,y0,z0)(x-x0) + Fy(x0,y0,z0)(y-y0) + Fz(x0,y0,z0)(z-z0) = 0, where Fx, Fy, and Fz are the partial derivatives of the below F(x,y,z): 1. F(x,y,z) = ln(x+z) - yz 2. F(x,y,z) = yz - ln(x+z) When I take the partial derivatives Fx,Fy,Fz for both of these functions, they turn out to be different: For (1.), I get: Fx = 1/(x+z) Fy = -z Fz = 1/(x+z) - y For (2.), I get: Fx = -1/(x+z) Fy = z Fz = y - 1/(x+z) , which all have the opposite signs of (1.) So my question is: how do I know which F(x,y,z) to use when trying to find the tangent plane through a particular point?