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# Edexcel C2 20th May 2015 *Official Thread* watch

1. (Original post by enaayrah)
4th line:

Sn-rSn= a- ar^n

Yep!

BTW, is the bit I've highlighted right?

(Original post by Lynku555)
People, I think the proof to a geometric series may come up...

Sn = a + ar + ar^2 + . . . . . . . . ar^n-1
rSn = ar + ar^2 + ar^3 + . . . . . . ar^n + ar^n+1

Sn - rSn to eliminate repeated values

Sn - rSn = a + (ar^n)

Factor out Sn and a to get:

(1 - r)Sn = a ( 1 - r^n)

Divide through by (1 - r):

Sn = a (1 - r^n) / (1 - r)

Why do I think this will come up?

I feel like this paper will makes us do a lot of writing.

Proving the series, stating whether a value is a minimum or a maximum, or if an area is an under/over estimation.

If I did this proof wrong, or you notice a little mistake, please correct me!
2. How do we know if the area of trapezium is an overestimate or an underestimate?
3. (Original post by LearningGeek)
where can you access the reserved papers for past years?
http://www.physicsandmathstutor.com/...rs/c2-edexcel/
4. (Original post by leoben49)
How do we know if the area of trapezium is an overestimate or an underestimate?
Really steep curve over estimate really flat is underestimate
Tjat how i usually do it

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5. Look at you little Midgardians - worrying pathetically over a silly exam. I will make you cower in the shadow of the mighty binomial expansion, while the trigonometric equations attack your puny calculators.
6. (Original post by leoben49)
How do we know if the area of trapezium is an overestimate or an underestimate?
I think that if the area of the trapezium does not account a bit of the actual area below the curve it is an under-estimate.

And vice-versa
7. (Original post by a.a.k)
Guys share something that is tricky or rare like proving sum to infinity etc

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Is this on the spec?

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8. (Original post by Lynku555)
Yep!

BTW, is the bit I've highlighted right?
For the rSn line, don't put the last term (ar^n+1) in, it just complicates things (and it can't really be a part of Sn if you think about it?)
9. (Original post by Azzer11)
Is this on the spec?

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I dont know

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10. (Original post by leoben49)
How do we know if the area of trapezium is an overestimate or an underestimate?
If the curve is similar to a U shape then its an overestimate as the trapezium would be over the curve however if is similar to an upside down U shape then the trapezium would be under the curve so its an underestimate
11. (Original post by InkHeart)
Look at you little Midgardians - worrying pathetically over a silly exam. I will make you cower in the shadow of the mighty binomial expansion, while the trigonometric equations attack your puny calculators.
Hulk smash calculators hulk eat calculator buttons

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12. guys please i don't understand the very last question (( https://c4a3f001dcd45afe69d0ceec8300...%20Edexcel.pdf
13. (Original post by Azzer11)
Is this on the spec?

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Nope
14. (Original post by Azzer11)
Is this on the spec?

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yes it is
15. (Original post by enaayrah)
For the rSn line, don't put the last term (ar^n+1) in, it just complicates things (and it can't really be a part of Sn if you think about it?)
Good point!

I just get confused as to how ar^n - 1 minus ar^n cancel out idk why
16. Hey guys, just a quick question if anyone can help on Solomon Paper E for C2 on Q8.

MS:https://b940eb267c31463d4d23ccfd3105...%20Edexcel.pdf

I don't get how the mark scheme gets an answer of:
8(b)
end of 8th year: 500 × (1.06 + 1.062 + 1.063 + ... + 1.068) = 500 × S8;
a = 1.06, r = 1.06
B1 = 500 × 8 1.06[(1.06) 1]1.06 1−− M1
A1 = 5245.66 ∴ £5246 (nearest pound)

Here's the question:

Amy plans to join a savings scheme in which she will pay in £500 at the start of each year. One scheme that she is considering pays 6% interest on the amount in the accountat the end of each year.

(b) show that after interest is paid at the end of the eighth year, the amount in the account will be £5246 to the nearest pound.
17. (Original post by particlestudent)
I am so hoping it is as easy as June 2013 R, oh please
I just did that paper lel

No circle questions on it tho..

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18. (Original post by Azzer11)
Is this on the spec?

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I think so, with the proof for Sn of a geometric series.
It's just:
Sum to infinity = a/1-r
n tends towards infinity
r^n tends towards 0 when 0<r<1

Sn = a(1-r^n)/1-r so sum to infinity = a(1-0)/1-r = a/1-r
19. (Original post by The Q maestro)
I just did that paper lel

No circle questions on it tho..

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But it was easy right? I found it so easy
20. (Original post by ahony004)
yes it is
Wtf different people are saying different things? We never got taught it...

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