# Directional Derivative

1. Oct 7, 2008

### freecw

1. The problem statement, all variables and given/known data
Find the directional derivative of f at P in the direction of a.
f(x,y)=y2lnx; P(1,4); a= -3i + 3j

2. Relevant equations

3. The attempt at a solution
I first normalized the vector and got an = -3/(18)^1/2 + 3/(18)^1/2
Then i took the partial derivative and multiplied it by the vector
y2/X(-3/181/2) + 2(y)lnx(3/181/2
Ln(1) = 0 so the whole right side is gone, and after i plug in 1 and 4 on the right side i get -16/21/2. The answer should be -8/21/2. I've done 5 problems like this now and each time i'm off by a negative or a factor or something like that. What am i doing wrong?

2. Oct 7, 2008

### Dick

I get -16/sqrt(2) as well. Are you sure the answer wasn't -8*sqrt(2)? That would be the same thing you have.

3. Oct 7, 2008

### freecw

Thanks, i just went back and checked the back of the book, the answer is supposed to be -8*sqrt(2) not -8/sqrt(2). Also i didn't notice that -16/sqrt(2) is the same thing as -8*sqrt(2). Thanks for the help.