Question

Q.15

Answer 1

Given A.P. is `17,15,13,............`

Here, 

`"first term", a=17` 

Common difference, `d=15-17=2`

Let there be n terms in this A.P. 

Then ,` S_n=72`

`⇒ n/2[2xx17+(n-1)xx(-2)]=72`

`⇒n/2[34-2n+2]=72`

`⇒n[36-2n]=144`

`⇒2n^2-36n+144=0`

`⇒n^2-18n+72=0`

`⇒ n^2-12n-6n+72=0`

`⇒n(n-12)-6n+72=0`

`⇒(n-12)(n-6)=0`

`⇒n=12 or n=6`