Posted by **jack** on Tuesday, June 18, 2013 at 1:50pm.

S=1+2(1/5)+3(1/5)^2+4(1/5)^3….

If S=a/b, where a and b are co prime positive integers, what is the value of a+b?

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**Count Iblis**, Tuesday, June 18, 2013 at 1:53pm
S(x) = 1 + x + x^2 + x^3 +... = 1/(1-x)

dS/dx = 1 + 2 x + 3 x^2 + 4 x^3 +...

= 1/(1-x)^2

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**jack**, Tuesday, June 18, 2013 at 2:12pm
what's the sum of infinite terms?

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**jack**, Tuesday, June 18, 2013 at 2:16pm
thanks i have now understood the problem.............

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**Abhishek Ranjan**, Wednesday, June 19, 2013 at 3:59am
41

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