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# c1: arithmetic series watch

1. The first term of an arithmetic series is 14
Common difference = -6, and so is the sum of the first n terms.

Find n.

Could someone please explain how to calculate this? I tried to get two equations so that I could solve them simultaneously, but failed.

I know this a really stupid question...but it's stumped me.
2. What is an expression for the sum of the first n terms?
3. Sn = n/2 [2a+(n-1)d]

A = 14, D = -6.

Sub it in.
4. Question:
what does N stand for?
l-last
d-difference
so whats n?
5. (Original post by v1oXx-)
Sn = n/2 [2a+(n-1)d]

A = 14, D = -6.

Sub it in.

Unless I miscalculated that would gives me 0=30n-6n^2-12, which doesn't give me the answer.
6. Course it does.

Factorize.
7. (Original post by v1oXx-)
Course it does.

Factorize.
It won't factorise;
6(-2 + 5n + -1n^2) is what it looks like factorised.
8. Try 2 brackets?
9. (Original post by v1oXx-)
Try 2 brackets?
Because you can't.

Anything else that I could do to solve this?
10. ive just skimmed this so you might have done it already and rearranged but shouldnt you be setting it equal to -6 rather than 0?

as d = Sn = -6

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