f=9-x^2-y^2 and u=i-j(adsbygoogle = window.adsbygoogle || []).push({});

The directional derivative comes out to be Du f(x,y)=-sqrt(2)+sqrt(2)

I'm going to find the directional derivative and could someone kindly point out the mistake because Im getting a different answer and it's important I understand how to do this question:

Du f(x,y) is simply the ∇f.u (note u is a vector)

Now ∇f=-2xi-2yj and ∇f.u=(-2xi-2yj).(i-j) = -2x+2y. Help?

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# Help required for Directional derivatives

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