Let f:R rarr R^(*) be a derivable functi...

Let `f:R rarr R^(*)` be a derivable function such that `f(x+y)=3f(x)f(y),AA x,y in R` with `f(1)=(2)/(3)` ,then `sum_(r=1)^(oo)(1)/(f(r))` is equal to

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