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If the sum of first 7 terms of an A.P. is 49 and that of 17 terms is 289, find the sum of first n terms.

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asked Mar 14, 2018 in Mathematics by shabnam praween (19,050 points) ● 5 ● 6 ● 8
If the sum of first 7 terms of an A.P. is 49 and that of 17 terms is 289, find the sum of first n terms.

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answered Mar 14, 2018 by mdsamim (213,225 points) ● 5 ● 10 ● 15
selected Mar 20, 2018 by faiz
 
Best answer

⇒ a + 8d = 17                           ..(ii)

Now, subtracting equation (i) from (ii), we have

5d = 10 ⇒ d = 2

Putting the value of d in equation (i), we have

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