# Homework Help: Improper Integrals - converge or diverge

1. Mar 17, 2010

### zeion

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

Hello. I have some questions on this assignment, I'm wondering if I could get some help:
Determine whether the integral converges and, if so, evaluate the integral.

1) $$\int_{e}^{\infty} \frac{dx}{xlnx}$$

2) $$\int_{1}^{4} \frac{dx}{x^2 - 4}$$

3) $$\int_{e}^{\infty} \frac{dx}{(lnx)^2}$$

4) $$\int_{2}^{\infty} \frac {dx}{x^2 + sinx}$$

2. Relevant equations

3. The attempt at a solution

1) I integrate and get $$\lim_{b \to \infty} \int_{e}^{b} \frac{dx}{xlnx} = \lim_{ b \to \infty} [ \frac{ln(xlnx)}{lnx}]_{e}^{b}$$ ?

2) It has discontinuity at x = 2 and x = -2 so I evaluate $$\int_{1}^{2} \frac{dx}{x^2 - 4} \int_{2}^{4} \frac{dx}{x^2 - 4}$$

I integrate and get $$\frac{-1}{4} \lim_{c \to \infty} \int_{1}^{c} \frac{1}{x+2} - \frac{1}{x-2} dx$$

I sub in and get ln0?

3) Does this integrate into $$\lim_{b \to \infty} [\frac {-x}{lnx}]_{e}^{b}$$ ?

4) I don't know how to integrate this
1. The problem statement, all variables and given/known data

2. Relevant equations

3. The attempt at a solution

2. Mar 17, 2010

### LCKurtz

Your antiderivative is incorrect. Try u = ln(x)

Don't you need a + sign between them?

Where did $c \rightarrow \infty$ come from? Your intgerals are improper at x = 2.
No.

To integrate 1/(x-a) try a u substitution.