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# Arithmetic progressions watch

1. The sum of the first n terms of a series is n(n+2). Find the first three terms of the series.

I don't have a clue! All I've got is sn=n(n+2)
2. (Original post by Lucofthewoods)
The sum of the first n terms of a series is n(n+2). Find the first three terms of the series.

I don't have a clue! All I've got is sn=n(n+2)
Use the formula for the term of the first n terms of a series, and you're given to find the first three terms so when n=3. Put it in your formula to get the answer.
3. (Original post by Lucofthewoods)
The sum of the first n terms of a series is n(n+2). Find the first three terms of the series.

I don't have a clue! All I've got is sn=n(n+2)
Let the nth term of your series be given by and the sum of the first terms be .
Note that: .

You know a formula for . So if you sub in you can find .

What can you say about and its relation to and ?
4. (Original post by Lucofthewoods)
The sum of the first n terms of a series is n(n+2). Find the first three terms of the series.

I don't have a clue! All I've got is sn=n(n+2)
Call your arithmetic progression . , , , . The partial sums of this progression, aka the sum of the first n terms, is found by adding up the first n terms of our arithmetic progression. this can be compacted by using sigma notation . We are told what the sum of the first terms of our progression is . So,

Notice, that when

You now need to find, and . Good luck, I hope what I've said helps.

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