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Joint Entrance Examination
Mathematics
Sequences and Series
Question 307
Question 307 of 519
Easy
MCQ
Let
a
1
,
a
2
,
…
,
a
10
a_1,a_2,\ldots,a_{10}
a
1
,
a
2
,
…
,
a
10
be a G.P. If
a
3
a
1
=
25
\dfrac{a_3}{a_1}=25
a
1
a
3
=
25
, then
a
9
a
5
\dfrac{a_9}{a_5}
a
5
a
9
equals:
A
5
4
5^4
5
4
B
4
(
5
2
)
4(5^2)
4
(
5
2
)
C
5
3
5^3
5
3
D
2
(
5
2
)
2(5^2)
2
(
5
2
)
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Solutions · 1
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Official solution
a
3
a
1
=
r
2
=
25
⇒
r
=
±
5
\dfrac{a_3}{a_1}=r^2=25 \Rightarrow r=\pm5
a
1
a
3
=
r
2
=
25
⇒
r
=
±
5
. \[\frac{a_9}{a_5} = r^4 = 5^4.\]
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