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## Homework Statement

[tex]D_{u}(f)(a,b) = \triangledown f(a,b)\cdot u[/tex]

[tex]D_{(\frac{1}{\sqrt2}, \frac{1}{\sqrt2})}(f)(a,b) = 3 \sqrt{2}[/tex]

where [tex]u = (\frac{1}{\sqrt2}, \frac{1}{\sqrt2})[/tex]

find [tex]\bigtriangledown f(a.b)[/tex]

## Homework Equations

## The Attempt at a Solution

first you change grad f into it's partial derivative form and then take the dot product:

(df/dx, df/dy).(1/root2, 1/root2) = 3root2

you'll find that:

df/dx + df/dy = 6

where would I go from here? quite confused.