Sunday 13 January 2019

real analysis - To find the sum of the series $,1+ frac{1}{3cdot4}+frac{1}{5cdot4^2}+frac{1}{7cdot4^3}+ldots$




The answer given is $\log 3$.
Now looking at the series



\begin{align}
1+ \dfrac{1}{3\cdot4}+\dfrac{1}{5\cdot4^2}+\dfrac{1}{7\cdot4^3}+\ldots &=
\sum\limits_{i=0}^\infty \dfrac{1}{\left(2n-1\right)\cdot4^n}
\\
\log 3 &=\sum\limits_{i=1}^\infty \dfrac{\left(-1\right)^{n+1}\,2^n}{n}
\end{align}




How do I relate these two series?


Answer



Hint: a common series that is used for computing log of any real number is
$$
\log\left(\frac{1+x}{1-x}\right)=2\left(x+\frac{x^3}3+\frac{x^5}5+\frac{x^7}7+\dots\right)
$$
$u=\frac{1+x}{1-x}\iff x=\frac{u-1}{u+1}$


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