Homework Statement
S is the surface with equation z = x^2 +2xy+2ya) Find an equation for the tangent plane to S at the point (1,2,9).
b) At what points on S, in any, does S have a horizontal tangent plane?
The Attempt at a Solution
F(x,y,z): z = x^2 +2xy+2y
F_x = 2x + 2y
F_y = 2x + 2...