# Math Help - Derivatives Problem

1. ## 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.

2. Just take the derivative of your 1st function and set it equal to 4 and replace x with r. Solve for r.

3. 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?

4. 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 $\sqrt{2} \, x^2$ with respect to x is $2 \sqrt{2} \, x$.

5. Originally Posted by mr fantastic
No, the derivative of $\sqrt{2} \, x^2$ with respect to x is $2 \sqrt{2} \, x$.
Thank you. I'm new I need to figure out how to get problems to show up so well with the math tags.

6. 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.
http://www.mathhelpforum.com/math-he...-tutorial.html