Arithmetic Progressions
> Question
LA
4 marks
Year: 2020
Medium
CBSE Class 10
arithmetic-progressions
sum
nth-term
Question
Solve:
1
+
4
+
7
+
10
+
…
+
x
=
287
Solution
1
x
=
a
n
=
1
+
3
(
n
−
1
)
=
3
n
−
2
S
n
=
287
⇒
2
n
[
1
+
3
n
−
2
]
=
287
+2 marks
2
3
n
2
−
n
−
574
=
0
(
n
−
14
)
(
3
n
+
41
)
=
0
⇒
n
=
14
+1 mark
3
∴
x
=
3
n
−
2
=
40
+1 mark
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