Question

If the sum of first 7 terms of an AP is 49 and that of 17 terms is 289.find the sum of first n terms.​

Answer 1

Answer:

sn=n/2(2a+(n-1)*d

s7=7/2(n-1)*7

s7=3.5*n-1*7

49-7=3.5*n-1

420/35=n-1

12+1=n

13=n