f(x,y) = x**2 + 3*x*y + 2y**2 dfx(x,y) = 2*x + 3*y dfy(x,y) = 3*x + 4*y a = -3; b = 2; A = dfx(a,b); B = dfy(a,b); C = f(a,b); g(x,y) = C + A*(x-a) + B*(y-b) splot f(x,y) , g(x,y) , 0 pause -2