Question

Q.11

Answer 1

Sum of first n term=`S_n=5n^2-8n`

`⇒ S_1=5(1)^2-8(1)=5-8=-3=a=t_1`

Also, `S_2=5(2)^2-8(2)=20-16=4`

`⇒t_1+t_2=4`

`⇒-3+t_2=4`

`⇒t_2=7` 

Now, `t_2-t_1=7-(-3)=7+3=10=d`

∴ Required A.P. `=a, a+d, a+2d,..........`

                        =-3,7,17............

`∴ 15^(th) "term" = t_15=-3+14xx10=-3+140=137`