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

1. ## 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?

$\int \frac {\cos x}{x} dx$

2. The antiderivative cannot be expressed in terms of elementary functions.

3. The question is

Suppose for a certain function f it is known that

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

Use integration by parts to evaluate

$\int_\frac{\pi}{2}^\frac{3\pi}{2} f(x) dx$

4. $\int_\frac{\pi}{2}^\frac{3\pi}{2} f(x) dx = xf(x) \Big|^{\frac{3\pi}{2}}_{\frac{\pi}{2}} - \int^{\frac{3\pi}{2}}_{\frac{\pi}{2}} xf'(x) \ dx$

$= xf(x) \Big|^{\frac{3\pi}{2}}_{\frac{\pi}{2}} - \int^{\frac{3\pi}{2}}_{\frac{\pi}{2}} cos(x) \ dx$

5. ## 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.

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

7. 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.

8. Will do.