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lgs98jonee
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#1
Report Thread starter 16 years ago
#1
Find the sum of the first n terms of the series
4 + 44 + 444 + 4444 + 44444 + ....

all i can get is

n
(n-k)*4*10^k
k=0
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Jonny W
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#2
Report 16 years ago
#2
(sum over 0 <= k <= n) r^k = [1 - r^(k + 1)] / (1 - r).

Differentiate wrt r to get an expression for

(sum over 0 <= k <= n) k*r^(k - 1).

Then put r = 10 into both sums.
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