# Homework Help: Error in taking the derivative of an integral

1. Feb 22, 2006

### jesuslovesu

I know that it's 6x^2 - 2 but when I'm trying take the derivative of the integral shouldn't I have to multiply each term by -1 because the x is in the lower bound? It gives a wrong answer, so am I doing something wrong or is it just that I'm not supposed to take the opposite in this case?

$$$\int_x^{-1} (2-6t^2)\,dt$ -1 * 2(-1 - x) - -1*6*(-1-x^3)/3$$

Last edited: Feb 22, 2006
2. Feb 22, 2006

### topsquark

$$\int_x^{-1}(2-6t^2) dt$$
$$(2t -2t^3)|_x^{-1}$$

3. Feb 22, 2006

### topsquark

$$\int_x^{-1}(2-6t^2) dt$$
$$(2t -2t^3)|_x^{-1}$$
$$[2(-1)-2(-1)^3]-[2x-2x^3]$$
$$2x^3-2x$$

-Dan

4. Feb 22, 2006

### jesuslovesu

In this case unfortunately I have to do it the long way using

c*(b-a) and (b^3-a^3)/3

5. Feb 22, 2006

### topsquark

It's the same thing, just rearrange the terms:

$$(2t -2t^3)|_x^{-1} = 2(-1-x)-2[(-1)^3-x^3]$$

-Dan

6. Feb 23, 2006

### VietDao29

May I suggest you NOT to give out COMPLETE solutions??? :grumpy: :grumpy: :grumpy: :grumpy:

Share this great discussion with others via Reddit, Google+, Twitter, or Facebook