**The A.P. is given by 9, 17, 25, .........**

**First term, ***a = 9*

**Common difference, ***d = 17 - 9 = 25 - 17 = 8*

**Let ***n* be the number of terms for which the sum is 636.

**∴Sum of ***n* terms, *S*_{n} = 636

**As the number of terms can not be negative, therefore, ***n* = 12.