# Integral [example thread of why you should post all given info]

• Jun 11th 2009, 05:28 PM
chengbin
Integral [example thread of why you should post all given info]
This thread is now changed to ask people to remember to post the whole question instead of one part. It saves time and confusion.

Thank you.

Original question (look at this as an example)

How do you integrate this function?

$\displaystyle \int \frac {\cos x}{x} dx$
• Jun 11th 2009, 05:32 PM
Random Variable
The antiderivative cannot be expressed in terms of elementary functions.
• Jun 11th 2009, 05:43 PM
chengbin
The question is

Suppose for a certain function f it is known that

$\displaystyle f'(x)=\frac {\cos x}{x}$, $\displaystyle f(\frac {\pi}{2})=a$ and $\displaystyle f(\frac {3\pi}{2})=b$

Use integration by parts to evaluate

$\displaystyle \int_\frac{\pi}{2}^\frac{3\pi}{2} f(x) dx$
• Jun 11th 2009, 06:12 PM
Random Variable
$\displaystyle \int_\frac{\pi}{2}^\frac{3\pi}{2} f(x) dx = xf(x) \Big|^{\frac{3\pi}{2}}_{\frac{\pi}{2}} - \int^{\frac{3\pi}{2}}_{\frac{\pi}{2}} xf'(x) \ dx$

$\displaystyle = xf(x) \Big|^{\frac{3\pi}{2}}_{\frac{\pi}{2}} - \int^{\frac{3\pi}{2}}_{\frac{\pi}{2}} cos(x) \ dx$
• Jun 12th 2009, 03:10 AM
mr fantastic
Why it's important to post the whole question, not just the bits that seem important
This is an excellent thread for showing why it's important to post the whole question, not just the bits of the question that seem important.
• Jun 12th 2009, 03:14 AM
chengbin
I kind of wanted to do the question myself, and ask for help only on the part I'm stuck on.
• Jun 12th 2009, 03:24 AM
mr fantastic
Quote:

Originally Posted by chengbin
I kind of wanted to do the question myself, and ask for help only on the part I'm stuck on.

Then you should post the whole question, show what you've done, say where your stuck and then state that you want only a hint rather than the whole solution.
• Jun 12th 2009, 07:31 AM
chengbin
Will do.