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# Integration by Parts [SOLVED]

Registered User regular
Ugh. I don't even remember if this was in Calc I or II.

Anyway, I'm integrating exp(-Dt)dt. If I set u=exp(-D), v=exp(t), I get exp(-Dt). If I set u=1, x=exp(-Dt), I get -1/D*exp(-Dt). Those are not the same. Why aren't they the same?

Oh, good luck, ceres!

SkyEye on

## Posts

• not beautiful like you Pennsylvania, USASuper Moderator, Moderator mod
It's Calc I. I have it now. The final is next week. Please kill me.

The avalanche has already started; it is too late for the pebbles to vote.
• Got catz in my zone Ca-catz in my zoneRegistered User regular
so you're integrating e^(-Dt) dt or e^(-Dt) dD?

also, this might help

twit | make stuff | GW2: Arithmetock.3459 | a list of wishes | a post of wishes | svpply
• Registered User regular
Oh, sorry. The differential term is dt. I'm pretty sure the correct answer is the second one but I can't figure out why the first one doesn't work.

• Got catz in my zone Ca-catz in my zoneRegistered User regular
ok, so you're integrating e^(-Dt) dt where D is a constant

why are you integrating by parts?

twit | make stuff | GW2: Arithmetock.3459 | a list of wishes | a post of wishes | svpply
• Registered User regular
Hey, just wondering - if:

u = exp(-D)
v = exp(t)

Then

uv = exp(-D) * exp(t) = exp(-D + t) =/= exp(-Dt)

?

• Registered User regular
SkyEye wrote: »
Ugh. I don't even remember if this was in Calc I or II.

Anyway, I'm integrating exp(-Dt)dt. If I set u=exp(-D), v=exp(t), I get exp(-Dt). If I set u=1, x=exp(-Dt), I get -1/D*exp(-Dt). Those are not the same. Why aren't they the same?

The product of exp(-D) and exp(t) is exp(-D + t).

EDIT: Oops, eeccccoo beat me to it. Guess I should actually read the other replies before making my own

• Registered User regular
Goddamn, I knew this would make me look stupid somehow.

• Registered User regular
SkyEye wrote: »
Goddamn, I knew this would make me look stupid somehow.

Nah, don't fuss.

You're not the first person to have made this mistake.

Besides, now that you've made the mistake publicly, you'll remember when it comes to your exam!