(adsbygoogle = window.adsbygoogle || []).push({}); 1. The problem statement, all variables and given/known data

(Q) The derivative of f(x,y) at Po(1,2) in the direction i + j is 2sqrt(2) and in the direction of -2j is -3. What is the derivative of f in the direction of -i - 2j? Give reasons for your answers.

2. Relevant equations

The directional derivative is given by the formula:

∂f/∂x i+∂f/∂y j

3. The attempt at a solution

You get simultaneous equations when you apply the above equation and you find that

∂f/∂y = 3/2.

And ∂f/∂x = [4sqrt(2) - 3] / 2.

Then applying the dot product of this and -i - 2j, you get [-3-4sqrt(2)] / 2 but the answer is supposed to be -7/sqrt(5). How did they get that??

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# Homework Help: Directional derivative formula

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