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SammyS is offline
Oct23-11, 07:20 PM
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Quote Quote by shubox View Post
1. The problem statement, all variables and given/known data
∫(dx/((1+x^2)^2) from 0 to ∞
Determine whether the improper integral converges and if so, evaluate it.

2. Relevant equations
1+ tan^2(x) = sec^2(x)
1/sec(x) = cos(x)

3. The attempt at a solution
Initially I had no idea how to approach this problem. The answer in the back of the book is ∏/4, which tells me that maybe trig integrals are involved. So i started off with:
lim(R→∞) ∫(dx/((1+x^2)^2) from 0 to R.
lim(R→∞) ∫(dx/((1+tan^2(x))^2) from 0 to R.
=lim(R→∞) ∫(dx/(sec^4(x)) from 0 to R.
I do not know where to go from here. Any help would be appreciated
It's not a good idea to use the same variable in your substitution.

Let x=tan(θ) then dx = sec2(θ) dθ .

After finding the anti-derivative in terms of θ, change back to x, plug in the limits of integration, then take the limit.