This expression includes a sum of geometric progression.
For a G.P.
a0, a1, ... , an
where a0 * r = a1, a1 * r = a2, ...
then
Sn = a0(1-r^n)/(1-r) if r
or
Sn = a0(r^n-1)/(r-1) if r>1
=> 2 + 2 + 2^2 + 2^3 + 2^4 + 2^5 + 2^6 + 2^7 + 2^8 =
=> 2 + [2 + 2^2 + 2^3 + 2^4 + 2^5 + 2^6 + 2^7 + 2^8]
=> 2+[2(2^8-1)/2-1]
=> 2+2(2^8 - 1)
=> 2 + 2^9 -2
=> 2^ 9
I hope this helps.