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# C2 HELPpppppppppppppp watch

1. i was wondering if someone could explain to me something geometric series

ok so the formula for the nth term = ar^(n-1)

in some questions let say find the amount in the 8th year i thought you would do

the 8th term = ar^(8-1) yet this is not the case

can someone please explain when u minus the one and when you don't.

cheerr
2. (Original post by edwin8030)
i was wondering if someone could explain to me something geometric series

ok so the formula for the nth term = ar^(n-1)

in some questions let say find the amount in the 8th year i thought you would do

the 8th term = ar^(8-1) yet this is not the case

can someone please explain when u minus the one and when you don't.

cheerr
Hiya, I really dont understand this either so any help from anyone would be a great help!
3. if u are given a series.
Like the third term is 30 and the fifth term is 75.

Ar(3-1) = 30 so ar² = 30
Ar(5-1) = 75 so ar4 = 75
4. (Original post by edwin8030)
i was wondering if someone could explain to me something geometric series

ok so the formula for the nth term = ar^(n-1)

in some questions let say find the amount in the 8th year i thought you would do

the 8th term = ar^(8-1) yet this is not the case

can someone please explain when u minus the one and when you don't.

cheerr
It all depends on the situation.

If you are doing interest and you want to know how many years you have to wait to get some much cash with the interest paid at the end of the year and the money inserted at the start then I would say the first term is (original sum) x interest rate. This is like a x r, but really it is the first term "a" which is why sometimes it appears that people stop at arn-1 because they take "a" to be this value of the interest at the end of the first year.

So if you always want to use arn then make sure you always use "a" as the amount of cash originally put in (or equivalent in any other practical context).

If this seems crazy to anyone I could be wrong on this. Widowmaker or someone could you verify please?

On final request: Please be more specific with thread titles in future.

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