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JEE Main Math Class 11 Sequences and Series Part 3 Questions
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© examsnet.com
Question : 36
Total: 101
Let
S
n
denote the sum of the first
n
terms of an arithmetic progression. If
S
10
=
390
and the ratio of the tenth and the fifth terms is
15
:
7
, then
S
15
−
S
5
is equal to:
[1-Feb-2024 Shift 2]
800
890
790
690
Validate
Solution:
👈: Video Solution
S
10
=
390
10
2
[
2
a
+
(
10
−
1
)
d
]
=
390
⇒
2
a
+
9
d
=
78
t
10
t
5
=
15
7
⇒
a
+
9
d
a
+
4
d
=
15
7
⇒
8
a
=
3
d
From (1) & (2)
a
=
3
&
d
=
8
S
15
−
S
5
=
15
2
(
6
+
14
×
8
)
−
5
2
(
6
+
4
×
8
)
=
15
×
118
−
5
×
38
2
=
790
© examsnet.com
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