an = 1+ 2 + 2^2 + 2^3 + ...+ 2^(n-1) = 2^n -1
sn = (2^1 -1) + (2^2-1) + ...+ (2^n -1)
= (2^1+2^2 +..+ 2^n) - (1+1 +...+1)
= 2^(n+1) -2 -n