# Derivatives Problem

• Oct 8th 2008, 01:47 PM
JBknights10
Derivatives Problem
I have the calculator portion of my Ch. 3 Calculus test tomorrow, titled Derivatives.

I'm doing a review sheet right now, and am stuck on a problem.

If f(x) = 13 - 8x + √2x² + 1
and f '(r) = 4
find r.
• Oct 8th 2008, 02:42 PM
11rdc11
Just take the derivative of your 1st function and set it equal to 4 and replace x with r. Solve for r.
• Oct 8th 2008, 03:20 PM
JBknights10
I think I messed up taking the derivative of √2x² because f ' (x) came out to be like -6

What is the derivative of √2x² is it just 2?
• Oct 8th 2008, 03:42 PM
mr fantastic
Quote:

Originally Posted by JBknights10
I think I messed up taking the derivative of √2x² because f ' (x) came out to be like -6

What is the derivative of √2x² is it just 2?

No, the derivative of $\displaystyle \sqrt{2} \, x^2$ with respect to x is $\displaystyle 2 \sqrt{2} \, x$.
• Oct 8th 2008, 04:01 PM
JBknights10
Quote:

Originally Posted by mr fantastic
No, the derivative of $\displaystyle \sqrt{2} \, x^2$ with respect to x is $\displaystyle 2 \sqrt{2} \, x$.

Thank you. I'm new I need to figure out how to get problems to show up so well with the math tags.(Headbang)
• Oct 8th 2008, 07:29 PM
mr fantastic
Quote:

Originally Posted by JBknights10
Thank you. I'm new I need to figure out how to get problems to show up so well with the math tags.(Headbang)

http://www.mathhelpforum.com/math-he...-tutorial.html