#AS level arithmetic sequence q

15 messages · Page 1 of 1 (latest)

urban comet
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ignore some of the writing, anyone got any idea? this is AS level math.

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lilac birch
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i see something like what i want but just to be on the same page

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how would you find the sum of terms of an arithmatic sequence

urban comet
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one of these 2 formulas depending on what i have

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sn = 1/2n (a+l)

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or

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sn = 1/2n(2a+(n-1)d)

lilac birch
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this is the sum of the terms from 1 to n right

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and so you just plug in 10 for the first 10 terms, so that bits easy

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the issue is the last 10

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now if you know there are k terms (in the question)

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then you can actually write the sum of the last 10

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as the sum of the first k terms, minus the sum of the first k-10 terms