math
posted by jack .
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?

S(x) = 1 + x + x^2 + x^3 +... = 1/(1x)
dS/dx = 1 + 2 x + 3 x^2 + 4 x^3 +...
= 1/(1x)^2 
what's the sum of infinite terms?

thanks i have now understood the problem.............

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