# Using the integral test to test for divergence/convergence

1. Oct 13, 2011

### miglo

1. The problem statement, all variables and given/known data
$$\sum_{n=1}^{\infty}\frac{8\arctan{n}}{1+n^2}$$

2. Relevant equations

3. The attempt at a solution
so im comparing it to the integral $\int_{1}^{\infty}\frac{8\arctan{x}}{1+x^2}$
but at first i need to show that the function im integrating is continuous, positive and decreasing. I know its continuous and positive from 1 to infinity but i need to show that it is decreasing
so i found the derivative of the function and got $\frac{8-16x\arctan{x}}{(1+x^2)^2}$ but i got stuck trying to find the critical points
specifically i forgot how to solve equations like $8-16x\arctan{x}=0$, mainly just because of that extra x in front of the arctan
i know this boils down to more of a precalc problem but i posted this on the calc homework help because i thought maybe some of my earlier steps were wrong

2. Oct 13, 2011

### Staff: Mentor

I don't see anything wrong with your work.

The equation you're trying to solve can be simplified to 2x arctan(x) = 1. I don't know of any ways to get analytic solutions to this equation, but approximation techniques yield solutions at $x \approx \pm 0.765379$, according to WolframAlpha. From the graph here, you can say that 1 - 2xarctan(x) < 0 for all x > 1, hence the derivative you've found is negative for x > 1.

3. Oct 13, 2011

### miglo

yeah i tried wolfram alpha, but it didnt show any steps on how to solve that equation, was looking for an analytic solution but this will have to do
thanks mark44