# integration: by parts

• Jul 11th 2007, 07:33 AM
integration: by parts
$\int e^x(\cos{x})dx$

substituted it to

dv = e^(x) dx
v = e^(x)

u = (cos(x))
u = -sin(x) dx

and this one goes infinity for me
help needed for what to sub ... i dunno how the integrator got a short answer
• Jul 11th 2007, 07:47 AM
ThePerfectHacker
Quote:

$\int e^x(\cos{x})dx$

substituted it to

dv = e^(x) dx
v = e^(x)

u = (cos(x))
u = -sin(x) dx

and this one goes infinity for me
help needed for what to sub ... i dunno how the integrator got a short answer

Give this integral a name,
Let $X=\int e^x \cos x dx$

After doing integration by parts you get,
$X = \mbox{ something } + \int e^x \sin x dx$

After doing integration by parts again you get,
$X = \mbox{ something } + \mbox{ more something }- \int e^x \cos x dx$

And it seems it goes on forever. But $\int e^x \cos x dx = X$

So,
$X = \mbox{ something } + \mbox{ more something } - X$

Thus,
$2X = \mbox{ something } + \mbox{ more something }$

Thus,
$X = \frac{ \mbox{ something }+\mbox{ more something }}{2} + C$
• Jul 11th 2007, 08:06 AM