Integration by parts

**Jaffa** · March 30th 2009, 12:13 PM

I am struggling with this question
a(t)= t/10e^t/10 so v(t) I think is the integral of this?
can somebody explain this to me using integration by parts

**e^(i*pi)** · March 30th 2009, 12:58 PM

Originally Posted by Jaffa

I am struggling with this question
a(t)= t/10e^t/10 so v(t) I think is the integral of this?
can somebody explain this to me using integration by parts

Do you mean $a(t) = \frac{t}{10}e^{\frac{t}{10}}$ ? v(t) would be the integral is v and a were velocity and acceleration respectively.

I would substitute $b = \frac{t}{10}$ but it doesn't really matter.

therefore $\frac{db}{dt} = 0.1$ and $dt = 10db$

$a(t) = be^b$ hence $v(t) = \int{be^b \cdot 10db}$

Using integration by parts letting $u = b$ and $\frac{dv}{db} = e^b$ so that

$\frac{du}{db} = 1$ and $v = e^b$

$I = uv - \int{v\frac{du}{db}} = be^b - \int{e^b} = e^b(b-1)$

remembering that $b = \frac{t}{10}$ we get

$e^{\frac{t}{10}}(1-\frac{t}{10}) + C$

**Soroban** · March 30th 2009, 05:04 PM

Hello, Jaffa!

$\int \frac{t}{10}e^{\frac{t}{10}}dt$

We have: . $\frac{1}{10}\int t\, e^{\frac{t}{10}} dt$

I'll do it in conventional terms . . .

. . $\begin{array}{ccccccc}u &=&t & & dv &=& e^{\frac{t}{10}}dx \\ du &=& dx & & v &=& 10e^{\frac{t}{10}} \end{array}$

Then: . $\frac{1}{10}\bigg[t\cdot10e^{\frac{t}{10}} - \int10e^{\frac{t}{10}}dt\bigg] + C \;=\;\frac{1}{10}\bigg[10te^{\frac{t}{10}} - 100e^{\frac{t}{10}}\bigg] + C$

. . . $= \;e^{\frac{t}{10}}(t-10) + C$

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