help please

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February 6th 2006, 09:19 PM
kdstalnaker

help please

find the interval e^x (1+e^x)^2 dx
February 6th 2006, 09:48 PM
CaptainBlack

Quote:

Originally Posted by kdstalnaker

find the interval e^x (1+e^x)^2 dx

What is the derivative of:

$\frac{(1+e^x)^3}{3}\ ?$

RonL
February 7th 2006, 02:04 PM
ThePerfectHacker

Quote:

Originally Posted by kdstalnaker

find the interval e^x (1+e^x)^2 dx

I understand it as,
$\int e^x(1+e^x)^2dx$ .
Okay, call $u=1+e^x$ , then by substitution,
$\int u^2du=\frac{1}{3}u^3+C$
Thus,
$\frac{1}{3}e^{3x}+C$
Q.E.D.
February 7th 2006, 08:19 PM
CaptainBlack

Quote:

Originally Posted by ThePerfectHacker

I understand it as,
$\int e^x(1+e^x)^2dx$ .
Okay, call $u=1+e^x$ , then by substitution,
$\int u^2du=\frac{1}{3}u^3+C$
Thus,
$\frac{1}{3}e^{3x}+C$
Q.E.D.

Try differentiating this, then compare it with the integrand. They are
not the same. Then look at my posting; its a hint. Differentiate what
I suggest and you will recover the integrand. So what does that tell
you about the integral?

RonL

RonL
February 8th 2006, 06:23 PM
ThePerfectHacker

Quote:

Originally Posted by CaptainBlack

Try differentiating this, then compare it with the integrand. They are
not the same. Then look at my posting; its a hint. Differentiate what
I suggest and you will recover the integrand. So what does that tell
you about the integral?

RonL

RonL

Not sure where you are getting at?
February 8th 2006, 08:47 PM
CaptainBlack

Quote:

Originally Posted by ThePerfectHacker

Not sure where you are getting at?

What I am getting at is that

$<br /> \frac{d}{dx}\left(\frac{1}{3}e^{3x}+C \right)=e^{3x} \ne e^x(1+e^x)^2<br />$

and so $\frac{1}{3}e^{3x}+C$ is not the integral of $e^x(1+e^x)^2$

But:

$<br /> \frac{d}{dx}\frac{(1+e^x)^3}{3}\ = e^x(1+e^x)^2<br />$

and so $\frac{(1+e^x)^3}{3}+C$ is the integral of $e^x(1+e^x)^2$

RonL
February 9th 2006, 12:39 PM
ThePerfectHacker

Of course! Look at how I used substitution rule. I subsituted $e^x$ for $u$ instead of $1+e^x$ .

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