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January 3rd, 2009, 10:26 AM   #1
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Chain rule questoin

Hello all,

I'm stuck on a problem and I'm hoping someone can help. Here is the problem:

If y = f((x^2 + 9)^1/2) and f '(5) = -2, find dy/dx when x = 4.

Here is what I have tried so far.

I know that dy/dx = dy/du * du/dx. If u = (x^2 + 9)^1/2, then du/dx = x/(x^2 + 9)^1/2.

so...
-2 = dy/du * x/(x^2 + 9)^1/2

If I substitute x = 5, then...
-2 = dy/du * 5/(34)^1/2

This doesn't seem right. I think I'm going down the wrong path to solve this one. Can anyone help me with this problem?

Thanks.
johngalt03 is offline  
 
January 3rd, 2009, 06:04 PM   #2
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Re: Chain rule questoin

Remember that when , , which is not necessarily equal to .
Scott is offline  
January 4th, 2009, 11:22 AM   #3
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Re: Chain rule questoin

and , find dy/dx when x = 4






Let x = 4





since f'(5) = -2:

CogitoErgoCogitoSum is offline  
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