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# C2/C12 Geometric Series help? watch

1. I am always uncertain when it comes to word problems, especially when it comes to questions that deals with to calculate amount of money in "n".
Sometimes you are supposed to use the formulae ar^n-1 and sometimes ar^n.
Is there any trick I can use to figure out what should the value of "n" ?
2. (Original post by Sayf200)
I am always uncertain when it comes to word problems, especially when it comes to questions that deals with to calculate amount of money in "n".
Sometimes you are supposed to use the formulae ar^n-1 and sometimes ar^n.
Is there any trick I can use to figure out what should the value of "n" ?
I find it easiest to check with a small number - like after 2 time units. So do it the longhand way and with the formulae and see which gives the right answer.
3. (Original post by Sayf200)
I am always uncertain when it comes to word problems, especially when it comes to questions that deals with to calculate amount of money in "n".
Sometimes you are supposed to use the formulae ar^n-1 and sometimes ar^n.
Is there any trick I can use to figure out what should the value of "n" ?
the formula of geometric series is always ar^n-1. can u help me with a question that confused u so I can help?
4. (Original post by brainmaster)
the formula of geometric series is always ar^n-1. can u help me with a question that confused u so I can help?
5. (Original post by ZaraHuss)
I find it easiest to check with a small number - like after 2 time units. So do it the longhand way and with the formulae and see which gives the right answer.
Well you couldn't possibly do that in an exam right? I mean you don't the the right answer.......
6. (Original post by Sayf200)
Well you couldn't possibly do that in an exam right? I mean you don't the the right answer.......
What i meant was do a quick check you're using the right one. You can quickly get the answer for after 1 year by the GCSE method and then check with the formula you've used. The correct formula will be right for 1 year as well as the many years you're probably calculating.
Alternately the reason use ar^(n-1) is that the first time you add interest is the second year, whereas if you include the first year as getting interest then it would be ar^n. This particularly comes up with money when you put the money into the bank before or after they give interest that year.
7. (Original post by ZaraHuss)
What i meant was do a quick check you're using the right one. You can quickly get the answer for after 1 year by the GCSE method and then check with the formula you've used. The correct formula will be right for 1 year as well as the many years you're probably calculating.
Alternately the reason use ar^(n-1) is that the first time you add interest is the second year, whereas if you include the first year as getting interest then it would be ar^n. This particularly comes up with money when you put the money into the bank before or after they give interest that year.
Aah, that will do for now. Thanks!

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