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# Arithmetic squence Watch

1. Sean pays a loan over a period of 'n' months. His monthly repayments from an arithmetic sequence.

He pays 149 in the first month, 137 in the second month, 145 in the third month, and so on. He makes his final repayment in the nth month, where n> 21

b) find the amount sean repaying in the 21st month

Over the n months, he repays a total of £5000

We know a = 149, d = -2 and sum = 5000 - what formula would i use or how would you approach this question?

- Thanks
2. I assume you use

nth term = a + (n-1) d
3. I tried that and you get

149 + (n-1)-2
149-2n+2
151=2n
n=76

Then i tried putting n into "Sum = n/2 [a+l]" to find the last term, but value was less than 25 :/
4. No, the value for the 21st month is:

=149 + (21-1) -2
=149 - 40
=109

Is that it or is there another part to the question?
5. nope - thats it.

Thanks P:

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Updated: December 21, 2010
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