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Mathematics
Mathematics
Problem
Determine the sum of the series
∑
n
=
3
∞
(
−
2
)
n
+
1
5
n
\displaystyle\sum_{n=3}^{\infty}\dfrac{(-2)^{\,n+1}}{5^{\,n}}
n
=
3
∑
∞
5
n
(
−
2
)
n
+
1
.
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