Question

if the sum of first seven terms of an AP is 49 and that of seventeen terms is 289 .find the sum of the first n terms.

Answer 1

Step-by-step explanation:

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

49=7(a+3d)

7=a+3d....(1)

S17=17/2(2a+16d)

289=17(a+8d)

17=a+8d.....(2)

on subtracting (1)&(2),we get

5d=10

d=2

putting d=2,in equ. (1),then we get

a=1

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

=n/2[2(n)]

=n2

sn =n2

Answer 2

do it further n u will get it's answer