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**Spec** At what angle does the tangent plane to the surface $\displaystyle S: z=2x^3-x^2y-y^2-2y+11$ in the point (2,3,0) meet the xy-plane?

$\displaystyle F=2x^3-x^2y-y^2-2y+11 - z=0$

$\displaystyle grad F(x,y,z) = (6x^2-2xy,-x^2-2y-2,-1)$

$\displaystyle grad F(2,3,0) = (12,-12,-1)$

The normal vector to the xy-plane, n, is (0,0,1).

$\displaystyle grad F \cdot n = |17||1|cos\theta$

$\displaystyle grad F \cdot n = -1$

So we have $\displaystyle cos\theta = \frac{-1}{17} \implies \theta = arccos \frac{-1}{17}$

The correct answer is $\displaystyle \theta = arccos \frac{1}{17}$ though, which is what you get if you select $\displaystyle (0,0,-1)$ as the normal vector. So are both answers correct? Why is it that you choose (0,0,-1) as the normal vector?