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# Multiple sums watch

1. ok so i dont get this question:

Suppose x>1. Write sum(i=1 to infinity) of ix^i as a double sum and evaluate by swapping the order of summation.

thanks
2. anyone?????
3. (Original post by victoria13)
ok so i dont get this question:

Suppose x>1. Write sum(i=1 to infinity) of ix^i as a double sum and evaluate by swapping the order of summation.

thanks
As no one's posted:

Unless I am missing something, if x>1 the sum does not converge.

So assuming you meant |x|<1; I can't see an elegant way to do this, so I'd write ix^i

as x^i + x^i +... + x^i ( i times in all) and put it into a grid horizontally (one line at a time) then look to the verticals, and you can get a double sum which you can evaluate.

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