# Math Help - integral (cos(x))/1+4sin^(2)x

1. ## integral (cos(x))/1+4sin^(2)x

I would be grateful if someone would have a look at the working on this link.I cant paste picture here or save in correct format and upload..
http://www.wolframalpha.com/input/po...&s=41&button=1

could someone please talk me thru this problem???
I understand to use u and du but am unsure where the values 1/2 and the number 2 in 2u come from.In my book it says to use a compensatory value.Im guessin here that 2 compensates 1/2 or is it the other way round? I will be using arctan rather than tan^-1.Any help will be great.Thanks

2. ## Re: integral (cos(x))/1+4sin^(2)x

Look at this.
Click the show steps button.

3. ## Re: integral (cos(x))/1+4sin^(2)x

$u = \sin{x} \implies du = \cos{x}dx$

That transforms the integral into $\int\frac{1}{1+4u^2}du$

4. ## Re: integral (cos(x))/1+4sin^(2)x

Thanks Plato.Yes thats where i tried initially but im still unsure about the numbers 2 and 1/2.Im a little lost with it......

5. ## Re: integral (cos(x))/1+4sin^(2)x

Thanks Ridley.Im a little unsure about using the numbers 2 and 1/2.Plus i cant see du in the solution.Many thanks.

6. ## Re: integral (cos(x))/1+4sin^(2)x

You can make another variable change if you want to.

$y = 2u$ gives us $dy = 2du$ and the integral

$\frac{1}{2}\int\frac{1}{1+y^2}dy=\frac{1}{2} \arctan{(y)}+C=$
$=\frac{1}{2}\arctan{(2u)}+C=\frac{1}{2} \arctan {(2\sin{x})}+C$

7. ## Re: integral (cos(x))/1+4sin^(2)x

but where does the cos x dissapear to in the equasion??? I must be having a really dumb day.
Thanks

8. ## Re: integral (cos(x))/1+4sin^(2)x

Could you show me where the cos x disapears to please???Thanks

9. ## Re: integral (cos(x))/1+4sin^(2)x

That terms disappears when you do this variable change: $u = \sin x \implies du = \cos x dx \implies dx = \frac{du}{\cos x}$

$\int \frac{\cos x}{1 + 4\sin ^2{x}}dx=\int \frac{\cos x}{1 + 4u^2}\cdot \frac{du}{\cos x}=\int\frac{1}{1+4u^2}du$

10. ## Re: integral (cos(x))/1+4sin^(2)x

thanks for your pointers.the arrangement for dx made things clear.youre a star.thanks again.