Question

the sum of first 7 terms of an ap is 49 and sum of the first 17 terms is 289 find the sum of first three terms​

Answer 1

Solution :

given that s7=49=72(sn=n2)

s17=289=172

now sn=n2[2a+(n−1)d]

s7=72[2a+(7−1)d]

49=72[2a+6d]

7=a+3d eqn(1)

now S17=172[2a+(17−1)d]

289=172[2a+16d]

a+8d=17 eqn(2)

subtracting equations (1) & (2)

we get 5d=10

d=2

puting it in eqn (1)

a+3(2)=7

a=1

sn=n2[2a+(n−1)d]

sn=n2[2+(n−1)2]

=n2[2+2n−2]

sn=n2

answer

Answer 2

Refers to attachment ♡~