Geometric Series is annoying me!
Hi,
Can someone please explain something about geometric series that is really p***ing me off?
So, for the question about the car, for part b they (in the mark scheme) have used arn but for part b of the question about yearly profit, they have used arn-1. Why are they doing this differently. It's really annoying because I have looked through my notes but just don't get it!
Thanks!
Can someone please explain something about geometric series that is really p***ing me off?
So, for the question about the car, for part b they (in the mark scheme) have used arn but for part b of the question about yearly profit, they have used arn-1. Why are they doing this differently. It's really annoying because I have looked through my notes but just don't get it!
Thanks!
Original post by 020200
Hi,
Can someone please explain something about geometric series that is really p***ing me off?
So, for the question about the car, for part b they (in the mark scheme) have used arn but for part b of the question about yearly profit, they have used arn-1. Why are they doing this differently. It's really annoying because I have looked through my notes but just don't get it!
Thanks!
Can someone please explain something about geometric series that is really p***ing me off?
So, for the question about the car, for part b they (in the mark scheme) have used arn but for part b of the question about yearly profit, they have used arn-1. Why are they doing this differently. It's really annoying because I have looked through my notes but just don't get it!
Thanks!
It depends on whether we take the first term to be year 0 or year 1, and also on the precise meaning of phrases like "in the 3rd year" and "after 3 years". So in the car question, if we take £18000 as the first term, that corresponds to "after 0 years" (as no time has passed since the car was first purchased), so £18000*0.8^1 would be "after 1 year", then £18000*0.8^2 would be "after 2 years", £18000*0.8^3 would be "after 3 years", and so on. Alternatively, if we take the first term to be "after 1 year", then it would be a=£18000*0.8, and after 3 years would be the third term, so using ar^(n-1) would give £(18000*0.8)*(0.8^2) = £18000*0.8^3 as before.
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