# Finding specified partial derivatives

1. Oct 24, 2007

### dalarev

1. The problem statement, all variables and given/known data

2. Relevant equations

3. The attempt at a solution

I follow the steps until I get to 2(x+y+z)(1 - sin(r+s) + cos(r+s))

The actual derivation process isn't the problem, I get lost in trying to figure out when to plug values for r and s.

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• ###### partialder.gif
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Last edited: Oct 24, 2007
2. Oct 24, 2007

### Dick

3. Oct 24, 2007

anyhow, you have done everything correctly thus far. just plug the given values for s and r into the final expression 2(x+y+z)(1 - sin(r+s) + cos(r+s))

you will never go awry if you always plug in given values last

4. Oct 24, 2007

### dalarev

I'm guessing this is after I replace x,y,z with their respective functions, correct?

edit: Negative. I did that and got 0 as an answer. According to the answer guide (back of the book), the correct answer is 13.

Last edited: Oct 24, 2007
5. Oct 24, 2007

yes, i'm sorry, i should have specified.... plug in x,y,z and then s, r

6. Oct 24, 2007

### dalarev

That's not correct though, I get 6(0) = 0, correct answer should be 13.

7. Oct 24, 2007

i'm currently unable to view the images you attached... what were x,y, and z again?

EDIT: i recall r and s being 1 and -1, right? if that's the case (1 - sin(r+s) + cos(r+s) should be equal to (1 - sin0 + cos0) = (1 - 0 +1) = 2, not zero..

Last edited: Oct 24, 2007
8. Oct 24, 2007

### dalarev

x = r - s
y = cos(r+s)
z = sin(r+s)

9. Oct 24, 2007