Homework Help: Integrals and differentials

1. Aug 25, 2008

fiziksfun

Can someone explain to me why $$\int\frac{x}{x^2+1}$$ = $$\frac{1}{2}$$ln(x^2+1) WHERE DOES THE ONE HALF COME FROM ???

2. Aug 25, 2008

Staff: Mentor

Re: Help

Take the derivative of the right hand side and find out for yourself. (Hint: Chain rule.)

3. Aug 25, 2008

snipez90

Re: Help

$$(ln(u))' = \frac{u'}{u}$$

4. Aug 25, 2008

fiziksfun

Re: Help

ok none of these responses helped me at all.

5. Aug 25, 2008

Staff: Mentor

Re: Help

Did you take the derivative of the right hand side?

6. Aug 25, 2008

NoMoreExams

Re: Help

You can also evaluate the LHS i.e. the integral

Hint #1 - it's a u-subst.

Hint #2 - let u = x^2 + 1

7. Aug 25, 2008

snipez90

Re: Help

I'm sorry but the OP has no business trying a u-sub if he doesn't follow the first hint. The idea that you can take the derivative to verify the correctness of an integral is fundamental. Remembering to apply the chain rule answers the question. I mean sure, he could apply a technique that really follows from these basic principles but then it would be hard to tell if he actually knew why.