Quick directional derivative question -- help please

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SUMMARY

The discussion focuses on calculating the directional derivative of the function f(x,y) = x(1+y)-1 at the point (0,0) in the direction of the vector u = (1, -1). The gradient of the function is determined to be grad f(x,y) = ((1+y)-1, -x(1+y)-2), yielding grad f(0,0) = (1, 0). The correct directional derivative is found to be 1/√2, as the unit vector in the direction of u must be used in the calculation, correcting the initial oversight.

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Homework Statement


[/B]
find directional derivative at point (0,0) in direction u = (1, -1) for

f(x,y) = x(1+y)^-1

The Attempt at a Solution



grad f(x,y) = ( (1+y)^-1, -x(1+y)^-2 )

grad f(0,0) = (1, 0)

grad f(x,y) . u = (1,0).(1,-1) = 1.

seems easy but markscheme says I am wromg. It says answer is 1/root2
 
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In the last line you need to take a unit vector in the direction of u.
 
physichu said:
In the last line you need to take a unit vector in the direction of u.

Duh! you're awesome. Thanks.
 

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