hi, just wanted some guidance on these problems.

so is this right..

using

that is the sum right?

also how would I solve for, i'm guessing there is also a trick for this..

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- Jul 28th 2008, 09:00 AMdankelly07sum of series help..
hi, just wanted some guidance on these problems.

so is this right..

using

that is the sum right?

also how would I solve for, i'm guessing there is also a trick for this..

- Jul 28th 2008, 09:49 AMgalactus
Yes, multiply your result from the first one by 9.

- Jul 28th 2008, 10:08 AMgalactus
If we let the original series be S and factor, we can get

and solve for S.

Factor out 9/10:

But what I have underbraced is

So what we get is

Solve for S=90 - Jul 28th 2008, 10:13 AMMoo
- Jul 28th 2008, 11:04 AMdankelly07
- Jul 28th 2008, 12:12 PMdankelly07
oops i think it was supposed to be 2/(1-x^3)

wasn't it? - Jul 28th 2008, 12:59 PMMathstud28
- Jul 28th 2008, 02:04 PMPlato
You know from above that .

The derivative is .

Multiply by x, .