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# Edexcel C4 Revision Thread (28/06 14:00) watch

1. (Original post by ABUSHOMAL///M3)
plz if a Q. asks for volume of a curve rotated 360 or 180 , what do we use
pie or 2pie for anyone plz answer me any1 now
rotated 360 degrees around x axis = π ∫ y² ∂x (relates to πr² )

and 180 degrees I assume is half of that
2. plz if a Q. asks for volume of a curve rotated 360 or 180 , what do we use
pie or 2pie for anyone plz answer me any1 now

plz very essential
3. (Original post by ABUSHOMAL///M3)
plz if a Q. asks for volume of a curve rotated 360 or 180 , what do we use
pie or 2pie for anyone plz answer me any1 now
pi for 360.

pi/2 for 180 (although I have never seen it on a paper).

Make sure you integrate the function squared, and if it is parametric this will involve: ∫y²(dx/dt)dt.

Also if it is round the y axis be sure to swap all x's with y's and vice versa. So the limits will have to y coordinates, and you'll have to integrate with respect to y etc.
4. (Original post by Widowmaker)
Well, i've just finished partial fractions, so I can help people with those, lol.
Although it's a very short/easy chapter so noone will need my help.
Easy but always on the exam papers for 5+ marks!
5. (Original post by samdavyson)
pi for 360.

pi/2 for 180 (although I have never seen it on a paper).

Make sure you integrate the function squared, and if it is parametric this will involve: ∫y²(dx/dt)dt.

Also if it is round the y axis be sure to swap all x's with y's and vice versa. So the limits will have to y coordinates, and you'll have to integrate with respect to y etc.
many thanks man>
6. (Original post by samdavyson)
pi for 360.

pi/2 for 180 (although I have never seen it on a paper).

Make sure you integrate the function squared, and if it is parametric this will involve: ∫y²(dx/dt)dt.

Also if it is round the y axis be sure to swap all x's with y's and vice versa. So the limits will have to y coordinates, and you'll have to integrate with respect to y etc.
if around y axis then x^2 dy ?
7. (Original post by ABUSHOMAL///M3)
if around y axis then x^2 dy ?
Yeah, there is a question on Edexcel's 2nd practice paper if you want a go.
8. (Original post by samdavyson)
Remember: am/n = nth root of a to the power of m.

So:

∫cosθ/(cos²θ )^3/2 dθ = ∫cosθ/(cos³θ) dθ = ∫1/(cos²θ) dθ

=> ∫sec²θ dθ => Standard Integral. ( => tanθ + C)
Thanks Sam.
So for the cos²θ^3/2 is it 2 x 3/2 to get 3? To make it cos³x.. Like multiplying the powers.. right?
9. (Original post by violet)
Thanks Sam.
So for the cos²θ^3/2 is it 2 x 3/2 to get 3? To make it cos³x.. Like multiplying the powers.. right?
Yeah exactly that.

Or you can think about it as the 2 in the 3/2 square roots the expression and the 2 in the cos²θ squares it. So overall ==> the twos cancel.

=> cos³x
10. (Original post by samdavyson)
Yeah exactly that.

Or you can think about it as the 2 in the 3/2 square roots the expression and the 2 in the cos²θ squares it. So overall ==> the twos cancel.

=> cos³x
I think I'll use the multiplying powers way... It sounds easier
11. (Original post by violet)
I think I'll use the multiplying powers way... It sounds easier
Fair enough. They are both the same thing.
12. (Original post by samdavyson)
Fair enough. They are both the same thing.
Oh, thanks anyway.

And also..... For Q5 on that paper, I can't get from the 2nd to last step to the last step in the answers.

I get V = pi[(8 + 1/4 ln4) + 3)] but how do you make that pi[5 + 1/2 ln2] ?
13. (Original post by violet)
Oh, thanks anyway.

And also..... For Q5 on that paper, I can't get from the 2nd to last step to the last step in the answers.

I get V = pi[(8 + 1/4 ln4) + 3)] but how do you make that pi[5 + 1/2 ln2] ?

When you have at minus 3:

V = pi[(8 + 1/4 ln4) - 3]

=> V = pi[(8 + 1/4ln2²) - 3]

=> V = pi[(8 + 1/2ln2) - 3]

=> V = pi(5 + 1/2ln2)

How was C3 by the way?
14. (Original post by samdavyson)

When you have at minus 3:

V = pi[(8 + 1/4 ln4) - 3]

=> V = pi[(8 - 1/4ln2²) - 3]

=> V = pi[(8 - 1/2ln2) - 3]

=> V = pi(5 - 1/2ln2)

How was C3 by the way?
How come you have to change from +1/4 to -1/4

It was ok i guess. I just scribbled out a lot and hope the examiner can still read it! How was it for you?
15. (Original post by violet)
How come you have to change from +1/4 to -1/4

It was ok i guess. I just scribbled out a lot and hope the examiner can still read it! How was it for you?
I thought the paper was alright.

I'd hope for an A on it.

You have a point there with the minus. I integrated it to give a minus, which is wrong. I will edit it in a second.
16. I think now I just need to learn my integrals (http://153rd.com/sam/as/C4/).

Then I should be ok.

Do you know them?
17. (Original post by samdavyson)
I think now I just need to learn my integrals (http://153rd.com/sam/as/C4/).

Then I should be ok.

Do you know them?
Do we need to know all the ones on that page?
I haven't done much C4 recently.. I did it ages ago and now I've forgotten it all and it suddenly seems so much harder!
18. (Original post by violet)
Do we need to know all the ones on that page?
I haven't done much C4 recently.. I did it ages ago and now I've forgotten it all and it suddenly seems so much harder!
Just the ones that aren't marked with the red line.

i.e. the first 6 lines.
19. (Original post by samdavyson)
Just the ones that aren't marked with the red line.

i.e. the first 6 lines.
I don't ever remember learning the first one ever
Lol, what is it?
20. (Original post by violet)
I don't ever remember learning the first one ever
Lol, what is it?
Add one to the power, and divide by the new power!

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