Can someone explain how I should be solving for theses terms? I got my series to be $10(-7x)^n$, but I dont know what to do from there.

Represent the function $f(x)=\frac{10}{(1-7x)}$ as a power series $$f(x)=\sum_{n=0}^\infty c_nx^n$$

Find the following coefficients:


Find the radius of convergence


  • $\begingroup$ Where does that "2" come from, in $10(-7(2))^0$? $\endgroup$ Apr 22, 2015 at 12:37
  • $\begingroup$ Use the binomial series for $g(y)=(1-y)^{-1}$ with $y=7x$ and them multiply through by $10$. The radius of convergence is then inherited from that of the series for $g(y)$ $\endgroup$ Apr 22, 2015 at 13:10

1 Answer 1


I think that in the preamble of your question you should have $10(7x)^n$ instead of the negative (this from Geometric series). It should be clear then that $c_n=10\cdot 7^n$. As far as the radius of convergence you should know that in the Geometric series, $|7x|<1$.


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