sum from m=2 to infinity of 6/(7^m)

解

m = 2 \sum \infty \frac{6}{7 ^{m}}

\frac{1}{7}

十進法表記

0.14285 \dots

解答ステップ

m = 2 \sum \infty \frac{6}{7 ^{m}}

定数乗算の規則を適用する:

\sum c \cdot a_{n} = c \cdot \sum a_{n}

= 6 \cdot m = 2 \sum \infty \frac{1}{7 ^{m}}

n = k \sum \infty (a_{n}) = n = 0 \sum \infty (a_{n}) - a_{0} - a_{1} - \dots a_{k - 1}

= 6 (m = 0 \sum \infty \frac{1}{7 ^{m}} - \frac{1}{7 ^{0}} - \frac{1}{7 ^{1}})

m = 0 \sum \infty \frac{1}{7 ^{m}} = \frac{7}{6}

= 6 (\frac{7}{6} - \frac{1}{7 ^{0}} - \frac{1}{7 ^{1}})

簡素化

6 (\frac{7}{6} - \frac{1}{7 ^{0}} - \frac{1}{7 ^{1}}) : \frac{1}{7}

= \frac{1}{7}