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Joint Entrance Examination
Mathematics
Sequences and Series
Question 335
Question 335 of 519
Medium
MCQ
If
b
b
b
is the first term of an infinite G.P. whose sum is
5
5
5
, then
b
b
b
lies in the interval:
A
(
−
∞
,
−
10
)
(-\infty,-10)
(
−
∞
,
−
10
)
B
(
10
,
∞
)
(10,\infty)
(
10
,
∞
)
C
(
0
,
10
)
(0,10)
(
0
,
10
)
D
(
−
10
,
0
)
(-10,0)
(
−
10
,
0
)
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Solutions · 1
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Official solution
b
1
−
r
=
5
⇒
b
=
5
(
1
−
r
)
\dfrac{b}{1-r}=5 \Rightarrow b=5(1-r)
1
−
r
b
=
5
⇒
b
=
5
(
1
−
r
)
. Since
−
1
<
r
<
1
-1<r<1
−
1
<
r
<
1
(for convergence),
b
∈
(
0
,
10
)
b\in(0,10)
b
∈
(
0
,
10
)
.
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