# Arithmeric seriesWatch

#1
Hi,
Can anyone explain how they got the answer to this question and also what formulas they used.

Mohammed starts a pension with £2500. Each year he deposits £500 more than the previous year. Find the total amount he will have depoaited after 30 years.

Thank you.
Last edited by Ohlala!; 1 week ago
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1 week ago
#2
(Original post by Ohlala!)
Hi,
Can anyone explain how they got the answer to this question and also what formulas they used.

Mohammed starts a pension with £2500. Each year he deposits £500 more than the previous year. Find the total amount he will have depoaited after 30 years.

Thank you.
nth term formula: U = a + (n-1)d, where a = 2500, d = 500
then to find the 30th term, U(30), U = 2500 + (30 - 1)500 = £17,000 which is a different answer to what they've got, I can't see why they've got £27,000.
Formula to Sum up to an nth term: Sn = n/2(a + l), where l = nth term (in the above case is 30th term which I think is £17000), a = first term = 2500
so S(30) = 30/2(2500 + 17000) = £292,500 (rather than £442,500 that they've got, which I think is wrong)
I can't see why there's a 49 multiplying the 500 in the first bit, I think it should be 29. Maybe someone else can see what I'm not seeing or they made a mistake in the answers but I'm pretty sure I haven't made any mistakes or missed anything
Last edited by auto-92; 1 week ago
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