Determine a formula for the sum of(adsbygoogle = window.adsbygoogle || []).push({});

[itex]\sum[/itex] ir^{i-1}

in terms of n and r.

I am stuck on this, i dont understand what to do with the "i" infront of the r^{i-1}

i know that [itex]\sum[/itex] r^{i-1}= (1-r^{n}) /1-r

all sums are for an index of i=1 to n

I just don't know how to go any farther. Any help is greatly appreciated. Thanks!

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# Difficult Summation Problem Grade 12

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