How can I integrate by parts this one?

In summary, the conversation discusses solving an integral with a function that does not have an antiderivative and the possibility of a typo in the problem. The suggested solution by one person may not be correct due to the function not strictly converging towards infinity and the possibility of a typo in the problem.
  • #1
glomar
5
0

Homework Statement



I have to solve the following integral

1/(sqrt[x] * ln[x]) from 2 to infinity

Homework Equations





The Attempt at a Solution



u= ln[x] dv=1/sqrt(x)
du= 1/x v= 2 sqrt(x)


If I do this I get

lim (ln b/ sqrt[x] - 4 sqrt[x] - ln2/sqrt[x] + 4 sqrt[x])

Is this the actual result of the integral? Did I substitute correctly?
Thanks in advance!
 
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  • #2
I don't think [tex] \int \frac{dx}{\sqrt{x}*ln(x)} [/tex] has an antiderivative.

Do you mean [tex] \int \frac{ln(x)}{\sqrt{x}}dx [/tex] ?
 
  • #3
glomar said:

Homework Statement



I have to solve the following integral

1/(sqrt[x] * ln[x]) from 2 to infinity

Homework Equations





The Attempt at a Solution



u= ln[x] dv=1/sqrt(x)
du= 1/x v= 2 sqrt(x)


If I do this I get

lim (ln b/ sqrt[x] - 4 sqrt[x] - ln2/sqrt[x] + 4 sqrt[x])

Is this the actual result of the integral? Did I substitute correctly?
Thanks in advance!

This problem can not be solved because the function is not strictly converging towards infinity. You can calculate definite integrals, f.ex. [2,100 000], [2, 1 000 000] and so on, but there is no way this integral reaches a certain limit as x goes towards infinity. I agree with Random Variable, this seems lik a typo.
 
  • #4
Probably it was an error from the Professor. I'll check that.
Thanks!
 

1. How do I know when to use integration by parts?

Integration by parts is typically used when you have a product of two functions in your integral. It can also be used to integrate functions that are the product of a polynomial and an exponential function.

2. What is the formula for integration by parts?

The formula for integration by parts is ∫udv = uv - ∫vdu, where u is the first function and dv is the second function.

3. How do I choose which function to use as u and which to use as dv?

There is no one set rule for choosing u and dv, but a common approach is to choose u as the function that becomes simpler when differentiated, and dv as the function that becomes easier to integrate.

4. What do I do if the integral becomes more complicated after using integration by parts?

If the integral becomes more complicated after using integration by parts, you can try using the formula again on the new integral. If this doesn't work, you may need to try a different integration technique.

5. Can integration by parts be used to solve all integrals?

No, integration by parts is just one of many integration techniques and it may not always work for every integral. It is important to be familiar with other integration methods as well.

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