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MrRa [10]
3 years ago
9

Find the sum of the convergent series 7 0.7 0.007

Mathematics
1 answer:
mafiozo [28]3 years ago
3 0
7+0.7+0.007+\cdots=7\left(1+\dfrac1{10}+\dfrac1{100}+\cdots\right)

Let S_n denote the nth partial sum of the series, i.e.

S_n=1+\dfrac1{10}+\dfrac1{100}+\cdots+\dfrac1{100^n}

Then

\dfrac1{10}S_n=\dfrac1{10}+\dfrac1{100}+\dfrac1{1000}+\dfrac1{10^{n+1}}

and subtracting from S_n we get

S_n-\dfrac1{10}S_n=\dfrac9{10}S_n=1-\dfrac1{10^{n+1}}
\implies S_n=\dfrac{10}9\left(1-\dfrac1{10^{n+1}}\right)

As n\to\infty, the exponential term vanishes, leaving us with

\displaystyle\lim_{n\to\infty}S_n=\dfrac{10}9

and so

7+0.7+0.007+\cdots=7.777\ldots=\dfrac{70}9
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