Question

If the sum of the first 7 terms of an AP is 49and that of 17 terms is 289,find the sum of first n terms
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Answer 1

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Answer 2

Step-by-step explanation:

S7=7/2(2a+(7-1)d)

49=7/2(2a+6d)

14=2a+6d

divide by 2 as all numbers given above are divisible by 2

7=a+3b eq 1

S17=17/2(2a+(17-1)d)

289=17/2(2a+16d)

34=2a+16d

divide by 2 as all numbers given above re divisible by 2

17=a+8d eq 2

From eq 1 and 2

7-3d =17-8d

8d-3d=17-7

5d=10

d=2

putting value of d in eq 1

a=7-3d

a=7-3(2)

a=7-6=1