# "3 hour intergral"?

• Jan 11th 2008, 12:13 PM
polymerase
"3 hour intergral"?
My prof was teaching a lecture then one thing lead to another and he talk about the fact that $\displaystyle \int \sqrt{sin\;x}\;dx$ DNE but however,$\displaystyle \int \sqrt{tan\;x}\;dx$ does! He followed by saying, "you can try at home to compute this 3 hour long integral":D Joke about the time, but I wondering how hard this really is?
• Jan 11th 2008, 12:29 PM
galactus
Look into Elliptic Integrals
• Jan 11th 2008, 12:41 PM
Krizalid
Quote:

Originally Posted by polymerase
$\displaystyle \int \sqrt{tan\;x}\;dx$ does! He followed by saying, "you can try at home to compute this 3 hour long integral":D Joke about the time, but I wondering how hard this really is?

Well, it depends of your integration skills.

By settin' $\displaystyle u^2 = \tan x$ you'll get another nasty integral, but it isn't if you work with some of make-up :D
• Jan 11th 2008, 12:46 PM
polymerase
Quote:

Originally Posted by Krizalid
but it isn't if you work with some of make-up :D

Wut exactly do u mean by make-up?
• Jan 11th 2008, 12:48 PM
Krizalid
I mean you can manipulate the integrand as you want to get a nice result.

The straightforward-nasty way leads to apply partial fractions (Puke)
• Jan 11th 2008, 01:01 PM
galactus
How I would attempt it is perhaps via the beta function....assuming you had limits of integration.

For instance,

$\displaystyle \int_{0}^{\frac{\pi}{2}}\sqrt{sin(x)}dx$

You could use $\displaystyle 2\int_{0}^{\frac{\pi}{2}}sin^{2p-1}(u)cos^{2q-1}(u)du$

With p=3/4 and q=1/2, we get:

$\displaystyle \frac{B(3/4,1/2)}{2}\approx{1.2}$

Just a little tidbit. Thought you may be interested.
• Jan 11th 2008, 06:22 PM
mr fantastic
Quote:

Originally Posted by polymerase
[snip]
$\displaystyle \int \sqrt{tan\;x}\;dx$ [snip] "you can try at home to compute this 3 hour long integral":D Joke about the time, but I wondering how hard this really is?

How hard ....? Only one way to find out, sport ..... try it!

You'll see it's not hard, just a bit long.

And after you've tried it, you can try $\displaystyle \int \frac{1}{\sqrt{tan\;x}} \;dx$.
• Jan 12th 2008, 02:42 PM
ThePerfectHacker
How about, $\displaystyle \int \sqrt{\sin \frac{1}{x} }~dx$?
(Hint: This integral is non-sense, why?)