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# OCR (not MEI) C2 - Wednesday 20th May 2015 watch

1. I got 3.3 rad but then took it away from 6 pi (graph is stretched by SF 3 on x-axis) and got 12 point something rad?
2. You know from the graph in the previous question that the point of intersection does not occur when sinx=0. Therefore you can divide by it as you know it can't be 0
3. For 9ii) I put (3/2pi - a) + 3/2pi instead of 3pi - a, how many marks would I lose?
4. (Original post by ayl21)
I got 3.3 rad but then took it away from 6 pi (graph is stretched by SF 3 on x-axis) and got 12 point something rad?
But the two graphs didn't intersect at the same point so you can't take it away from 6 pi???

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5. So if you solve for 3.3 rad you don't take it away from 6 pi because it's just symmetry?
6. (Original post by DominoMatrix227)
What did you get for the 2N thing? I got -44 and 125/3 which I think are both wrong.
I got 24

It was something like iirc. The sigma notation is indicating the sum of all the terms from n=N to n=2N which is the same as saying (just like the previous question where (because you'd be excluding if you did ).

So finding and simplifying:

Finding and simplifying:

This gets you brackets to expand:

very fiddly but the question depends on you being able to exploit the algebra

We can form a quadratic from this:

Then I used the quadratic formula, only bothering to calculate in the numerator because it was clear that using would mean that I would get a negative solution as well as a positive one, which wouldn't make sense in the context of sequences.

7. (Original post by ayl21)
So if you solve for 3.3 rad you don't take it away from 6 pi because it's just symmetry?
Sorry for sounding stupid, but how did you get 6pi in the first place and not 2pi?

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8. (Original post by jamesplay73)
For 9ii) I put (3/2pi - a) + 3/2pi instead of 3pi - a, how many marks would I lose?
That's literally exactly the same as 3pi-a... Did it not cross your mind to just add the 3/2 pi together? (For the record, you'll lose no marks but that's such a weird thing to write )
9. Could someone draw out the graph question drawing sin1/3 second last part on 9 . And draw out how they did the last question ? So confused would really appreciate it !!!! Xx
10. (Original post by Lil_timmi)
Sorry for sounding stupid, but how did you get 6pi in the first place and not 2pi?

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its a 1/3 x graph, hence stretch by SF3 in x direction. as it showed a full wave of cos on the diagram we know it must be 6pi not 2pi
11. (Original post by nurav11)
That's literally exactly the same as 3pi-a... Did it not cross your mind to just add the 3/2 pi together? (For the record, you'll lose no marks but that's such a weird thing to write )
Haha I did this too, the panic of exams does some very strange things...

12. This is how I answered 7. iii)

Can someone tell me where I went wrong???

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13. (Original post by nurav11)
That's literally exactly the same as 3pi-a... Did it not cross your mind to just add the 3/2 pi together? (For the record, you'll lose no marks but that's such a weird thing to write )
I know yea haha, I just had no time left thank you anyway
14. That might be necessary. I remeber completing it think what I have written doesnt look like 4 marks. But if you did that you would get the next answer right. When they say simplist form I am sure they mean without fractions. Everybody may have lost a mark if you are correct

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15. (Original post by andygeor)
its a 1/3 x graph, hence stretch by SF3 in x direction. as it showed a full wave of cos on the diagram we know it must be 6pi not 2pi
Ahhhh **** I got that wrong. Thanks anyway

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16. (Original post by nurav11)
That would be a godsend honestly, I got screwed over badly by the 2N question and could easily have lost 4 maybe even all 6 marks there. Apart from that though, thought it went very well, if boundary is 53 or something, then where do you reckon 100 UMS will be? 67/68? How'd you find it in general aswell?
Used my fp1 knowledge for that question :P. I thought it was a paper that required you to be careful, but otherwise was quite straight forward. I know i've lost all my marks on the last question cus of radians smh
17. (Original post by Lil_timmi)

This is how I answered 7. iii)

Can someone tell me where I went wrong???

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You can't just square the trig functions on each side and expect them to still equal eachother.
18. (Original post by Lil_timmi)

This is how I answered 7. iii)

Can someone tell me where I went wrong???

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Make it in terms of tanx dont square them, then remember the limits are larger because its 1/3

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19. Will we lose all 6 marks for getting N = -44 or N= 125/3 ?
20. I was considering this but when you are working to find solutions dont you do 360+ and 180- to get a set of solutions then whatever is combined with the x on the other side gets factored in to the solutions giving you the real set of solutions. The 1/3 may have only become relevant in the x intercept of the x axis for cos whatever it was. For example 2x=cos^-1(1/2), 2x=60, from here you would do 720-60 and 360+60.

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