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c1: arithmetic series

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.
Reply 1
Avatar for SimonM
SimonM
18
What is an expression for the sum of the first n terms?
Sn=n/2[2a+(n−1)d]Sn = n/2 [2a+(n-1)d]
Reply 3
Avatar for v1oXx-
v1oXx-
12
Sn = n/2 [2a+(n-1)d]

A = 14, D = -6.

Sub it in.
Reply 4
Avatar for jaz_jaz
jaz_jaz
Question:
what does N stand for?
l-last
d-difference
so whats n?
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.
Reply 6
Avatar for v1oXx-
v1oXx-
12
Course it does.

Factorize.
v1oXx-
Course it does.

Factorize.


It won't factorise;
6(-2 + 5n + -1n^2) is what it looks like factorised.
Reply 8
Avatar for v1oXx-
v1oXx-
12
Try 2 brackets?
v1oXx-
Try 2 brackets?


Because you can't.

Anything else that I could do to solve this?
Reply 10
Avatar for Rbates1
Rbates1
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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