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I'm having some trouble with this question: Prove that 0.1636363636...=9/55, using infinite series. I'd appreciate any help you can give me. Thanks!

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    I did take a quick glance at that one but was a little daunted by the complexity of the explanations.2011-05-13

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You can do this: \begin{align*} 0.1636363\cdots &= \frac{1}{10} + \biggl[ 63 \times 10^{-3} + 63\times 10^{-5} + 63 \times 10^{-7} + \cdots\biggr] \\ &= \frac{1}{10} + 63 \cdot \Bigl[ 10^{-3} + 10^{-5} + 10^{-7} + \cdots \Bigr] \\ &= \frac{1}{10} + 63 \cdot \frac{10^{-3}}{1-10^{-2}} \\ &= \frac{1}{10} + \frac{7}{110}=\frac{9}{55}\end{align*}

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    @Rasmus: By putting $\infty$ i actually meant that there are infinite number of terms. Anyhow thanks for the edit. It looks better without $\infty$.2011-05-13