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UPSC NDA 2 2012 Mathematics Paper
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© examsnet.com
Question : 5
Total: 120
What is the sum of first eight terms of the series
1
−
1
2
+
1
4
−
1
8
+
.
.
.
.
.
.
?
89
128
57
384
85
128
None of the above
Validate
Solution:
1
−
1
2
+
1
4
−
1
8
+
.
Sum of first '
n
' terms of G.P.
S
n
=
1
⋅
(
1
−
(
−
1
2
)
8
)
1
−
(
−
1
2
)
=
1
−
1
256
1
+
1
2
=
255
256
2
3
2
=
85
128
.
© examsnet.com
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