Sum of the first 20 terms of an AP is -240, and its first term is 7. Find its 24th term.
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Answer:
Let a be the first term and d be the common difference of the given A.P. It is given that
a
3
=7 and a
7
=3a
3
+2
a+2d=7 and a+6d=3(a+2d)+2
a+2d=7 and a=−1
a=−1,d=4
Therefore,
S
n
=
2
n
[2a+(n−1)d]
S
20
=
2
20
[2×−1+(20−1)×4]
S
20
=
2
20
(−2+76)=740
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