[tex] Sn= 1+ 2+ 4+ .... + 2^{n} \\
Sn = 2^{0}+2^{1}+2^{2}+ .... + 2^{n} /*2\\
2Sn= 2^{1}+2^{2} + ..... + 2^{n}+2^{n+1} \\ \\ \\
Obs. \ ca \ Sn -2^{0} = 2^{1} + 2^{2} + .... 2^{n} \rightarrow \\
2Sn= Sn - 2^{0} + 2^{n+1}/-Sn \rightarrow
Sn= 2^{n+1} - 1 [/tex]