//A1  = 1
//An = An-1 +sqrt(An-1)     for n > 1.

//i need to find An of the series according to the estimation An=(n^2)/4

//how do i write a member of the series, let's say "An" in the code and to apply "n>1" in the code?

This is where an if () statement comes in. It allows you to take different paths depending on a logical condition, like Is n greater than 1?

if (n>1)
    An = n*n/4; //n*n is n-squared.
    An = 1;