Hello

How do you integrate this, Integral (5x/3)*e^-x/3

Thank you

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- May 14th 2008, 11:03 AMmladenIntegral
Hello

How do you integrate this, Integral (5x/3)*e^-x/3

Thank you - May 14th 2008, 11:07 AMMoo
- May 14th 2008, 11:11 AMmladen
You refer to partiall integration?

F(x)g(x) - integral F(x)g(x)´

Så I should set F(x) = 5x/3 and g(x) = e^-x/3 ?

thank you - May 14th 2008, 11:14 AMMoo
Hm...

This is not the formula, but it looks like...

$\displaystyle \int_a^b f(x)g'(x) dx=\left[f(x)g(x)\right]_a^b - \int_a^b f'(x)g(x) dx$

And take $\displaystyle f(x)=5x/3$ and $\displaystyle g'(x)=e^{-x/3}$

(Sun)

Edit : your writing is confusing me... What is the original integral with f and g for you ? - May 14th 2008, 11:26 AMmladen
The original integral is "integral 5x/3*e^-x/3"

I set g(x) = e^-x/3

And f(x) = 5x/3

then used the formula

F(x)g(x) - integral F(x)g(x)´ - May 14th 2008, 11:33 AMMoo
Ok that's it :)

Set f(x)=e^-x/3

And g(x)=5x/3

Because g will be differentiated. Since the derivative of an exponential is in most case the same, it's not really useful to set g(x)=e^-x/3

Whereas if you differentiate a polynomial (5x/3), you will soon get to a constant.

Do you understand ?

(Wink) - May 14th 2008, 11:37 AMmladen
Yes I think I understand it now. Thanks alot =)