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UPSC NDA I 2011 Mathematics Paper
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© examsnet.com
Question : 12
Total: 120
Which term of the series
1
4
,
−
1
2
,
1
,
.
.
.
is
−
128
?
9th
10 th
11 th
12
th
Validate
Solution:
Since that given series are in G.P
Where first term
=
1
4
, common ratio
=
−
2
Let
−
128
is
n
th term
∴
−
128
=
1
4
⋅
(
−
2
)
n
−
1
⇒
−
128
×
4
=
(
−
2
)
n
−
1
⇒
(
−
2
)
9
=
(
−
2
)
n
−
1
n
=
9
+
1
=
10
th
© examsnet.com
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