# Evaluating improper intergral with trig function

1. Sep 6, 2015

### SteliosVas

1. The problem statement, all variables and given/known data

Okay so the problem is asking simply for a proof for convergence /divergence of the following indefinite integral:

∫(x*sin2(x))/(x3-1) over [2,∞)

2. Relevant equations

I know I can use substitution method
3. The attempt at a solution

Okay so I know if i factorize the bottom part of the integral I get:

∫(x*sin2(x))/(x-1)(x-1)(x2+x+1).

Now obviously I am not sure if this a very elemetary problem or indeed quite challenging. I have tried substituion making u = (x3-1) but than didn't really work... Not sure what else I should try

2. Sep 6, 2015

### showzen

I dont think that the problem wants you to solve the improper integral. Rather, it wants you to perform some test for convergence / divergence.

3. Sep 6, 2015

### SteliosVas

That's what I thought, as the computation of the improper integral would probably be too challenging... Should I use the limit test? In other words compare the overall function to the convergence/divergence of 1/x3 as it approaches 2

Last edited: Sep 6, 2015
4. Sep 6, 2015

### Ray Vickson

You don't need to worry about the lower limit x = 2, because nothing bad happens to the integrand $f(x) = x \sin^2(x)/(x^3-1)$ as $x \to 2$. However, you do need to worry about the upper limit as $x \to \infty$. Your textbook and/or course notes must have some relevant tests to use; try out some of them, then come back here if you still have some questions and issues.