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# OCR C4 (not mei) 18th June 2013 revision watch

1. (Original post by adam17)
you know cos4x = 2cos^2(2x) -1 from C3
rearrange to cos4x + 1 = 2cos^2(2x)
then 0.5cos4x + 0.5 = cos^2(2x)
Thank you

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2. i think someone else has asked for this question already but i don't understand why the answer to part 1 is 20/k1

i've attached the question

june 2009
Attached Images

3. (Original post by Hello_ImJess)
Help with question 4 please

Attachment 227518

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The reason why using is useful is because this helps us utilise the Pythagorean identity to simplify the denominator. This makes it easier to integrate because the denominator becomes much more simple.
4. (Original post by Hello_ImJess)
Help with question 4 please

Attachment 227518

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there you go, hope that's the right answer
5. Is it me or does C4 just loves putting little things to trip you up? Like the principle integration /differentiation etc isn't too bad but when you're setting up all the equations and you don't notice something cancels then you're screwed.
That's what I've been finding with the papers anyways.
6. (Original post by bbm100)
i think someone else has asked for this question already but i don't understand why the answer to part 1 is 20/k1

i've attached the question

june 2009
because its rising at K1 oC/s..

As its rising 20oC, it'll take 20/K1 seconds.

I don't really know how to explain it.. imagine K = 3.
To rise 20 degrees, it'll take 20/3 seconds.
7. (Original post by eggfriedrice)
Is it me or does C4 just loves putting little things to trip you up? Like the principle integration /differentiation etc isn't too bad but when you're setting up all the equations and you don't notice something cancels then you're screwed.
That's what I've been finding with the papers anyways.
after doing so many papers (which are all pretty similar) you sorta just learn the little tricks etc. That's what I've found anyway!
8. (Original post by bbm100)
i think someone else has asked for this question already but i don't understand why the answer to part 1 is 20/k1

i've attached the question

june 2009
Search it in youtube or search hegartydave and you should find a video on it
9. (Original post by bbm100)
i think someone else has asked for this question already but i don't understand why the answer to part 1 is 20/k1

i've attached the question

june 2009
The heater causes the temperature to increase at k degrees per second:

therefore

You can solve this differential equation with the information given and hense answer the question.

OR

Because temperature increases at a constant rate the rate of change of temperature is given by where t is the time taken for the increase. You may wish to think about this geometrically as a gradient of a straight line.
10. (Original post by stirkee)
after doing so many papers (which are all pretty similar) you sorta just learn the little tricks etc. That's what I've found anyway!
Haha yeah! I'm planning to do as many past papers today as I can and jot down all their little tricks. Sneaky C4.
11. (Original post by eggfriedrice)
Haha yeah! I'm planning to do as many past papers today as I can and jot down all their little tricks. Sneaky C4.
I'm pretty sure I know everything inside out.. my only downfall will be rushing and/or making stupid mistakes..

I find C4 easier than C3... I'm pretty sure I got over 90UMS in C3 so I'd hate myself if I screwed up my A* by mucking up C4.. lol.

but yeah I think (hope) i've seen all of the little tricks before!
12. (Original post by stirkee)
I'm pretty sure I know everything inside out.. my only downfall will be rushing and/or making stupid mistakes..

I find C4 easier than C3... I'm pretty sure I got over 90UMS in C3 so I'd hate myself if I screwed up my A* by mucking up C4.. lol.

but yeah I think (hope) i've seen all of the little tricks before!
Same actually, it took me a lot longer to understand C3 than C4. I got a B overall last year in Maths so I retook C1, C2, and C3 this exam season xD I'm predicted an A but now I'm greedy and want the A* >:
I love how M1/2 doesn't affect the A* though xD
13. (Original post by stirkee)

there you go, hope that's the right answer
I'm still a little confused. When you sub 1/3sin theta into 1 - 9x^2 you get 1-3sin^2theta. How did you get to the next line of working from there?

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14. (Original post by Hello_ImJess)
I'm still a little confused. When you sub 1/3sin theta into 1 - 9x^2 you get 1-3sin^2theta. How did you get to the next line of working from there?

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when you square 1/3sin theta you get 1/9sin^2theta.

multiply that by 9 to get sin^2theta.

1-sin^2theta = cos^2theta

get it?
15. (Original post by eggfriedrice)
Same actually, it took me a lot longer to understand C3 than C4. I got a B overall last year in Maths so I retook C1, C2, and C3 this exam season xD I'm predicted an A but now I'm greedy and want the A* >:
I love how M1/2 doesn't affect the A* though xD
I've found C3 much easier than C4 when learning it; its just since starting the past papers I've found C4 easier as the papers are all the same. I found that in C3 there's more random/hard stuff that they can throw at you but In C4 you know what is gonna come up every time.
16. (Original post by Hello_ImJess)
I'm still a little confused. When you sub 1/3sin theta into 1 - 9x^2 you get 1-3sin^2theta. How did you get to the next line of working from there?

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you get
17. (Original post by stirkee)
when you square 1/3sin theta you get 1/9sin^2theta.

multiply that by 9 to get sin^2theta.

1-sin^2theta = cos^2theta

get it?
Ahhhh got it, thank you

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18. How would you integrate this?

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19. (Original post by Hello_ImJess)
How would you integrate this?

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(integral of) x+2-2/x+2

= (integral of) 1 - 2/x+2

= x - 2ln(x+2)
20. (Original post by Hello_ImJess)
How would you integrate this?

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I'd use partial fractions, but in C4 you are not expected to be able to do partial fractions where the order of the denominator is the same as the numerator, so probably substitution?
If you said, let u=x+2, du/dx = 1 --> dx = du.
You then have to integrate (u-2)/u which you can split into integrate (1 - 2/u) du = u - 2lnu = x + 2 - 2ln(x+2).

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