## Direction derivitives

let f(x,y) be a differentiable function.Given that fx(2,4) =-3 and fy(2,4) =8, find F(2,4) where u is in the same direction of a =<2,4>
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 I didn't know. here is what i have so far..... i know that the gradient (F) is -3i +8j i think Duf(2,4) would be same i think vector a = <3,4> to unit would be <3,4>/5

## Direction derivitives

i think i got it. grad (f) doted with unit

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 Quote by antjm i think i got it. grad (f) doted with unit
That is correct!
Now, if your vector "a" was <2,4> as you wrote initially, that procedure is still the right one, but your unit vector would be somewhat different than if "a" was <3,4>.
Agreed?
 Agreed, a = <3,4> therefore magnitude would be 5 if a =<2,4> magnitude would be 2rt(5) thanks

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