The normal vector to the surface $z=10-2x-5y$ is given by the gradient of the function $f(x,y,z) = 10-2x-5y-z$. So, we have $\nabla f = \langle -2, -5, -1 \rangle$. However, we want the normal vector to point in the positive $z$-direction, so we take the negative
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