the sum of 5th and9th term is 40 and sum of 8th and 14th term is 64. find S 20
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Answer: 240
we know that Snth=N/2[2A+(N-1)D]
Step-by-step explanation:
Let a be the first term and d be the common difference of the AP.
Then, A/Q
(a + 4d) + ( a + 8d) = 40
a + 6d = 20 ......(i)
and, (14/2)(2a + 13d) = 64
2a + 13d = 64/7 ........(ii)
By solving equation (i) and (ii) , we get
a = 236/7
and d = -16/7
Therefore, Sum of first 20 terms are
(20/2)(2*(236/6) + 19(-16/7))
=240
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