# An Intergral Check

• Feb 24th 2010, 11:50 PM
Selim
An Intergral Check
$\int x^n sin(x) * dx$ = ______________________ + $\int x^{n-1} cos(x) * dx$

I want to just say sin(x), but that n should be a +1 and not minus or am I wrong? And,

$Evaluate \int 5x * sin(x) * dx$

Would you just pull the 5 out and call it a day or what?
• Feb 25th 2010, 01:16 AM
Pulock2009
Quote:

Originally Posted by Selim
$\int x^n sin(x) * dx$ = ______________________ + $\int x^{n-1} cos(x) * dx$

I want to just say sin(x), but that n should be a +1 and not minus or am I wrong? And,

$Evaluate \int 5x * sin(x) * dx$

Would you just pull the 5 out and call it a day or what?

in the blank cos(x) will come as it is the integration of sin(x). in ur second question as u said simply pull out 5 and integrate by parts.
• Feb 25th 2010, 01:34 AM
Prove It
Quote:

Originally Posted by Selim
$\int x^n sin(x) * dx$ = ______________________ + $\int x^{n-1} cos(x) * dx$

I want to just say sin(x), but that n should be a +1 and not minus or am I wrong? And,

$Evaluate \int 5x * sin(x) * dx$

Would you just pull the 5 out and call it a day or what?

Using integration by parts

Let $u = x^n$ so that $du = nx^{n - 1}$

Let $dv = \sin{x}$ so that $v = -\cos{x}$.

Then $\int{x^n\sin{x}\,dx} = -x^n\cos{x} - \int{-nx^{n - 1}\cos{x}\,dx}$

$= -x^n\cos{x} + n\int{x^{n - 1}\cos{x}\,dx}$.
• Feb 25th 2010, 01:40 AM
chabmgph
never mind