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How did you get through that one by the way?

I ended up getting to it like this.

$$ \nabla (\cos(f(x,y)))= \langle \frac{ \partial }{\partial x} \sin(f(x,y)), \frac{\partial}{\partial y} \sin(f(x,y)) \rangle$$ $$= \langle f_{x} \cos(x,y), f_{y} \cos(x,y) \rangle$$ $$=\cos(f(x,y)) \langle f_{x},f_{y} \rangle$$ $$=\cos(f(x,y) \nabla f(x,y)$$