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# Arithmetic progression help watch

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

can anyone explain how to do this??

2. What are you having difficulty with here? Substitue n=1, n=2 and n=3 into n(n+2) individually. I'll do n=1 for you...
n(n+2) = 1(1+2) = 3
3. That won't give the first, second and third elements straight off, you must do something with them.

f(n) = n(n+2)
f(1) = 3
f(2) = 8
f(3) = 15

Now, f(n) produces the sum of the first n terms.
So:
Sum of the first 1 term(s) = f(1) = 3
Sum of the first 2 terms = Second Term + f(1) = f(2) = 8
Sum of the first 3 terms = Third term + f(2) = f(3) = 15

Therefore:
First term = 3
Second Term = 8 - 3 = 5
Third term = 15 - 8 = 7

We can check this is right by suming the terms:
Sum of first 1 terms = 3 = 3
Sum of first 2 terms = 3 + 5 = 8 = 8
Sum of first 3 terms = 3 + 5 + 7 = 15 = 15

So we're right, it's:
3, 5, 7.

Therefore
4. thank you for your help we got two questions like that for homework

stuck on the other one now but ill give it a go
5. Therefore
... what ?

// -ve.
6. if my teacher tought it like that i would have had no problem lol

thanks for making it simple

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Updated: November 13, 2005
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