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

1. Can't believe I couldnt solve question 4 properly!!!
Mind went completely blank!
2. (Original post by ActaNonVerba)
-4sinxcosx + 2cosx = 0, 4sinxcosx = 2cosx, divide by cosx to get 4sinx = 2, then sinx=0.5..
You haven't disproved what I said. Given that there were three stationary points, you've actually made my point.
One SP was cos(x)=0. Standard C2 level question...

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3. (Original post by a10)
but cos x cant be zero the limits of x where greater than zero and less than pi? therefore u can divide by cos?

Pi/2
4. Proof there are 3 stationary points for those wondering:

5. (Original post by a10)
but cos x cant be zero the limits of x where greater than zero and less than pi? therefore u can divide by cos?
cosx=0 gives you pi/2 which is within the limits though. The limits are for x, not for cosx
6. For the balloon question, for inverse proportion I did 1/k multiplied by Sq. root of R, instead of k/Sq.root R. As a result, I got the answers wrong. But I did the correct integration etc. for the differential equation.

Does anyone know if I lost the whole 9 marks ? (( Or will I get error carried forward, and only lose about 2-3 marks? Thanks, I'm really scared, as I need an a*
7. (Original post by habs10)
For the balloon question, for inverse proportion I did 1/k multiplied by Sq. root of R, instead of k/Sq.root R. As a result, I got the answers wrong. But I did the correct integration etc. for the differential equation.

Does anyone know if I lost the whole 9 marks ? (( Or will I get error carried forward, and only lose about 2-3 marks? Thanks, I'm really scared, as I need an a*
I'd guess you should get 4/5 if you subbed in their values to give you a k and c
8. (Original post by tooambitious)
No, because it could be 0, can you divide by 0?

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Trueee,.. owned him
9. (Original post by tooambitious)
You haven't disproved what I said. Given that there were three stationary points, you've actually made my point.
One SP was cos(x)=0. Standard C2 level question...

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thought u were saying you couldn't take out cosx.. but yh, I only calculated 2 anyways, we're was the third?
10. (Original post by Holz888)
cosx=0 gives you pi/2 which is within the limits though. The limits are for x, not for cosx
Oh noooo I dropped that value, I completely forgot about it.

Basic error right there

I let myself down SO much in this exam. So many stupid mistakes caused by me just being stupid and losing my head.
11. (Original post by Namige)
Trueee,.. owned him
Don't mean to sound like a prick, just really surprised people are asking this question at C4 level, how did you do C2

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12. (Original post by habs10)
For the balloon question, for inverse proportion I did 1/k multiplied by Sq. root of R, instead of k/Sq.root R. As a result, I got the answers wrong. But I did the correct integration etc. for the differential equation.

Does anyone know if I lost the whole 9 marks ? (( Or will I get error carried forward, and only lose about 2-3 marks? Thanks, I'm really scared, as I need an a*
It doesn't matter, If you work out 1/k, it will still be the same as k if you did it properly. As it's just a constant.

For example

n = 1/5
1/n = 5

n=5
1/n=5
13. (Original post by Benjy100)
This is effectively a preliminary markscheme until Mr M posts his full definitive solutions. Some of my answers may be wrong. Spoilered incase you don't want to see them

*A few additions have been made to the markscheme* - namely the cartesian equation (forgot to put it in)

Spoiler:
Show
1.

2.

3. Skew. Solved the first two equations for and and then substituted into the third equation for 18=-38 which is clearly untrue, thus the equations are inconsistent and the lines are skew.

4.

Stationary points are

5.

Integral was

6.

7.

Angle BAC = 171.3 degrees (1 d.p.)

Show that AD is perpendicular to AB and AC by finding the dot product of AD and AB, and then AD and AC - and showing that both dot products equate to 0.

I messed up on the volume of the pyramid

8.

Letting t=0, finding r and substituting gives V = 30.5 cm^3 (3 s.f.)

9.

Solving dy/dx yields t=1 which corresponds to the point (0,3)

When t>1 x<0 and dy/dx is negative
When t<1 x>0 and dy/dx is positive

Hence it is a minimum

Cartesian equation...

10. Show.

Let x=0.1 for 0.136

Same substitution would be unsuitable as the expansion here is valid for which is the same as
Thankyou!! I forgot to plug x values back in to find y values for question 4.

Would it be okay to write 1/2ln(root3) ? Did they specify the format?

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14. (Original post by habs10)
For the balloon question, for inverse proportion I did 1/k multiplied by Sq. root of R, instead of k/Sq.root R. As a result, I got the answers wrong. But I did the correct integration etc. for the differential equation.

Does anyone know if I lost the whole 9 marks ? (( Or will I get error carried forward, and only lose about 2-3 marks? Thanks, I'm really scared, as I need an a*
They never specified that it be C/Sq.root R, so surely 1/(K Sq.root R) is still valid as 1/k is just a constant as is C. All this means is that you're final answer will look different to anyone else who did C/Sq.root R, but may still be a correct answer.

There's still hope!

However, if you did (1/k) * Sq.root R I would imagine you would lose some marks (such as for M1 at the start and the final answer mark), but I think you would get the rest of the marks for such a big question (i.e. error carried forward).
15. (Original post by Namige)
Trueee,.. owned him
lool calm your tits, I was clearly thinking of something different to you two.. I was just saying you can take it out of the equation to find values for x.. no need for the cheerleading
16. I forgot to sibstitute x back into u, would that cost me 1 or 3 marks?
17. (Original post by ActaNonVerba)
thought u were saying you couldn't take out cosx.. but yh, I only calculated 2 anyways, we're was the third?
Cos(x) = 0 where x=PI/2
Sin(x)=1/2 (iirc) where x= 5pi/6 or pi/6

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18. (Original post by zmai)
Thankyou!! I forgot to plug x values back in to find y values for question 4.

Would it be okay to write 1/2ln(root3) ? Did they specify the format?

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That's fine they said a ln b.. No indication that b must be an integer
19. (Original post by zmai)
Thankyou!! I forgot to plug x values back in to find y values for question 4.

Would it be okay to write 1/2ln(root3) ? Did they specify the format?

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Yup, agree with all apart from the volume, which you obviously didn't do lol
20. (Original post by h2shin)
I forgot to sibstitute x back into u, would that cost me 1 or 3 marks?
Mark scheme isnt that harsh. You'll probably lose 1 since it was the last step.

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