C3 Question

Hey, I done this question in the exam today and I want to know if I've gone somewhere wrong..

Integral of (t-1) lnt dt

limits= 3 and 1

u=lnt dv/dt=t-1
du/dx=1/t v=t^2/2 - t

=> (t^2/2-t)lnt - t^2/4 - t)

=> 3/2 ln3-21/4 + 5/4

=> 3/2 ln3-4

Did I go wrong somewhere? Please poin it out and sorry it is not that neat...

Thanks :smile:

Anyone ?

On this line:

=> (t^2/2-t)lnt - t^2/4 - t)

You've got one of your signs wrong. Up until then, however, it looks fine.

Reply 3

Shadow!

I got:

\int (t-1) lnt dt

u=lnt
du/dt= 1/t
dv/dt=t-1
v=t^2/2 - t

(\frac{t^2}{2} - t)lnt - \int \frac{t}{2} -1 dt

[(\frac{t^2}{2} - t)lnt - \frac{t^2}{4} + t]

Stick limits in to get:
1.5ln3 - 9/4 + 3 + 1/4 -1
1.5ln3 i think

I think you didn't do the integral of v du/dt in your integration by parts =[

But I'm severly lacking sleep so you may have to disregard that

Reply 4

_MaThs_

tommm

On this line:

=> (t^2/2-t)lnt - t^2/4 - t)

You've got one of your signs wrong. Up until then, however, it looks fine.

thanks

Reply 5

_MaThs_

Shadow!

I got:

\int (t-1) lnt dt

u=lnt
du/dt= 1/t
dv/dt=t-1
v=t^2 - t

(t^2 - t)lnt - \int t -1 dt

[(t^2 - t)lnt - \frac{t^2}{2} + t]

Stick limits in to get:
6ln3 - 9/2 + 3 + 1/2 -1
6ln3 - 2 i think

I think you didn't do the integral of v du/dt in your integration by parts =[

But I'm severly lacking sleep so you may have to disregard that

Thanks for your reply, it asked to integrate it by parts and the bold why isn't it t^2/2 - t?

Reply 6

Shadow!

_MaThs_

Thanks for your reply, it asked to integrate it by parts and the bold why isn't it t^2/2 - t?

I think I made a mistake but I'm too tired to double check =]

Reply 7

_MaThs_

Shadow!

I think I made a mistake but I'm too tired to double check =]

Hehe np, nite :p:

Reply 8

Shadow!

_MaThs_

Hehe np, nite :p:

hehe, night night to you too =]

Reply 9

Jooeee

Shadow!

1.5ln3 i think

I got this also in the exam today. I think it's right.

Reply 10

Shadow!

Jooeee

I got this also in the exam today. I think it's right.

Oh yeah =P

Tired maths ftw!

Reply 11

Jooeee

Shadow!

Oh yeah =P

Tired maths ftw!

Hmm being tired and doing maths does not work for me at all. The worst kind has to be doing drunk maths - DOES NOT WORK.

Spoiler

Reply 12

Shadow!

Jooeee

Hmm being tired and doing maths does not work for me at all. The worst kind has to be doing drunk maths - DOES NOT WORK.

Spoiler

Tell me about it, you're spot on there! I've done the same and it was just a mess. Went to a lecture drunk and scribbled over my mates work then realised it was mine :pinch: