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# The Edexcel C4 (21/06/12 - PM) Revision Thread Tweet

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IMPORTANT: You must wait until midnight (morning exams)/4.30AM (afternoon exams) to discuss Edexcel exams and until 1pm/6pm the following day for STEP and IB exams. Please read before posting, including for rules for practical and oral exams. 28-04-2013
1. Re: The Edexcel C4 (21/06/12 - PM) Revision Thread
how do we know when to multiply dy by dt/dy? I've seen it a few times but I always forget when i need to do that, is it when I change the limits?
2. Re: The Edexcel C4 (21/06/12 - PM) Revision Thread
when you guys integrate using substitution do you do it in sections or do everything at the same time?
3. Re: The Edexcel C4 (21/06/12 - PM) Revision Thread
(Original post by This Honest)
Thanks!!!

When you're adding and substracting logs, must the number be the same?

As in I can't do ln4 -0.5ln2 straight away, I have to make sure both terms are in ln2?
if you have to make it a single log, the question will explicitly state it
4. Re: The Edexcel C4 (21/06/12 - PM) Revision Thread
(Original post by James A)
ok,

so you get

∫(x/x+1)dx

u = x + 1

therefore x = u - 1

dx/du = 1

du = dx

hence replacing the integral with this info

∫(u-1)/u . du

∫(u/u)-(1/u) . du

∫ 1 - (1/u) . du

= u - ln(u) + C

I did that look above
But when I added my limits 0 and 3 I don't get what the MS says
Solomon paper E last question
5. Re: The Edexcel C4 (21/06/12 - PM) Revision Thread
(Original post by King_Arthur)
I love maths but it's still sickening and we had and M4 paper right at the beginning which was just impossible (actually impossible other than a few parts ) breaks down the moral stupid edexcel
Ahhh that sucks but dont worry you probably did finee.
damn.. i understand.
6. Re: The Edexcel C4 (21/06/12 - PM) Revision Thread
how do you know when to use substitution? like i only use it when they say, 'using the sub x= ...'
7. Re: The Edexcel C4 (21/06/12 - PM) Revision Thread
(Original post by excell)
Rates of change will probably come up now it wasn't in January
thats exactly what im thinking. I hope its not the end question.. cos it if its in the middle somewhere.. its not that bad.. Ahhhhhhh, i'm SO SO SO SO SO SCARED !!
8. Re: The Edexcel C4 (21/06/12 - PM) Revision Thread
(Original post by Mike-nificent)
emmm i don't really understand how to implicitly differentiate 3x2y

anyone??
product rule
bit like in c3 if we had (4x)sin(x)
so 3x^2(y)
= 6xy + 3x^2(dy/dx)
evertime you iff something with y you get dy/dx

hope that helps?
9. Re: The Edexcel C4 (21/06/12 - PM) Revision Thread
(Original post by excell)
Use the product rule.

u = 3x^2
v= y

(Original post by A128)
U gotta use the product rule so:

3x^2 dy/dx + 6xy
Why is the dy/dx on the x part. Not the y
10. (Original post by exe)
not me..

im gonna relax today. gonna do 4 papers chilledly and then try and relax for tomorrow... once you know all the techniques it's all about good sleep and retaining focus and getting into the right mindset
Totally agree with you (y) :d !
11. Re: The Edexcel C4 (21/06/12 - PM) Revision Thread
(Original post by CraziiFreak)
Yeahh i think so too. Not good
but still..
It sort of did come up in Jan 2012 for the last question though...
12. Re: The Edexcel C4 (21/06/12 - PM) Revision Thread
http://www.thomas-reddington.com/upl...p_jan_2012.pdf
can anyone explain how to do 3 b and c
13. Re: The Edexcel C4 (21/06/12 - PM) Revision Thread
(Original post by Dixie)
how do you know when to use substitution? like i only use it when they say, 'using the sub x= ...'
when you can solve it using the usaul methods

e.g you cant use the integration rule vdu = vu - ing(udv)
on ((sin^2)x)cosx
so you would have to use the subsition method
hope taht helps
14. Re: The Edexcel C4 (21/06/12 - PM) Revision Thread
15. Re: The Edexcel C4 (21/06/12 - PM) Revision Thread
(Original post by .Missy)
I dont get why we put the dy/dx on the 3x^2. Thought it was just on Y
Cause you differentiate y to make dy/dx and multiply it by the undifferentiated 3x^2 Product rule .
16. Re: The Edexcel C4 (21/06/12 - PM) Revision Thread
(Original post by Elle_6)
YOu cant integrate ln(x) like that you have to do 1 x ln(x) and do integration by parts.
The differentiation is correct though
So that will be xlnx-∫..
right?
17. Re: The Edexcel C4 (21/06/12 - PM) Revision Thread
What there be times where you have to make up your own substitution or would you always be provided it?
18. Re: The Edexcel C4 (21/06/12 - PM) Revision Thread
(Original post by This Honest)
I did that look above
But when I added my limits 0 and 3 I don't get what the MS says
Solomon paper E last question
just to confirm with you, was 3 the upper limit, right? and 0 being the lower limit?
19. Re: The Edexcel C4 (21/06/12 - PM) Revision Thread
(Original post by Mike-nificent)
http://www.thomas-reddington.com/upl...p_jan_2012.pdf
can anyone explain how to do 3 b and c
comparing cooefficeint
mutiply it all out
so 2(you expansion)+kx(your expansion) = the thing it says
compare coefficeints of x
so kx=7/4
you will get something like that
20. Re: The Edexcel C4 (21/06/12 - PM) Revision Thread
(Original post by 101101)
Why did this have to be an afternoon exam. ***** ********
agreed .... hate sitting with my thumb up my arse waiting to start the party
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