# Thread:

1. ## First-Order Partial Derivatives

Calculate the first-order partial derivative of the following:

for all (x,y) in $R^2$

I used this as a composition function and used the chain rule.

However, I am unsure of how to apply this formula.

First-order partial derivative of x would be

First-order pairtal derivative of y would be

My question is : how to compute this? Do I do another composition+chain rule?

Thank you.

2. Originally Posted by Paperwings
Calculate the first-order partial derivative of the following:

for all (x,y) in $R^2$

I used this as a composition function and used the chain rule.

However, I am unsure of how to apply this formula.

First-order partial derivative of x would be

First-order pairtal derivative of y would be

My question is : how to compute this? Do I do another composition+chain rule?

Thank you.
You cannot, it doesn't tell you what the function is. You have gone as far as you can

This is analgous to saying

.

Since in terms of the derivative in respect to x we don't care whether or not there are y's in there, this is just a normal chain rule of a function of x with some constants.

3. One more question: suppose that g has second derivative, how would I calculate the second partial derivatives of the function?

4. Hello,
Originally Posted by Paperwings
One more question: suppose that g has second derivative, how would I calculate the second partial derivatives of the function?
It depends on what second partial derivative you want.

You'll get the first one by taking the partial derivative of df/dx with respect to y. You'll get the second one by taking the partial derivative of df/dx with respect to x.

5. My book doesn't specify, but I'm pretty sure I'm supposed to find and . Thank you, Moo.

6. Originally Posted by Paperwings
My book doesn't specify, but I'm pretty sure I'm supposed to find and . Thank you, Moo.
Well then do what you did again

for $f_{xx}$ just hold y constant again and use either the chain rule and product rule or a combination of both if waranted. Same thing with [tex]f_{yy}[/math ] except now x is the variable held as a constant. if it is indeed $f_{xy}$ note that if hte curve is continuous that $f_{xy}=f_{yx}$ so you would only need to compute one.
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