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Mathematics
Mathematics
Problem
Let
S
=
2
+
6
7
+
12
7
2
+
20
7
3
+
30
7
4
+
…
S = 2 + \frac{6}{7} + \frac{12}{7^2} + \frac{20}{7^3} + \frac{30}{7^4} + \dots
S
=
2
+
7
6
+
7
2
12
+
7
3
20
+
7
4
30
+
…
. Then
4
S
4S
4
S
is equal to (a)
(
7
3
)
2
\left(\frac{7}{3}\right)^2
(
3
7
)
2
(b)
7
3
3
2
\frac{7^3}{3^2}
3
2
7
3
(c)
(
7
3
)
3
\left(\frac{7}{3}\right)^3
(
3
7
)
3
(d)
7
2
3
3
\frac{7^2}{3^3}
3
3
7
2
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