Question

in an AP if Sn=4n-n2 find the AP and common different

Answer 1

given ,

Sn =n ( 4n + 1 )

= 4n^2 + n

we know that,

Tn = Sn - S(n-1)

=4n^2+n -4 (n-1)^2 - (n-1)

=4 (n^2-n^2+2n-1)+(n-n+1)

=8n - 4 + 1

= 8n -3

hence ,

Tn = 8n -3

T1 =8 (1)-3 =5

T2= 8 (2)-3 =13

so, AP is 5, 13 , 21 and so on