${A}$ = ${1}$+${3}$+$3^{2}$+...+$3^{50}$
${3A}$ = ${3}$ . ( ${1}$+${3}$+$3^{2}$+...+$3^{50}$ )
${3A}$ = ${3}$+$3^{2}$+$3^{3}$...+$3^{51}$
${3A - A}$ = ( ${3}$+$3^{2}$+$3^{3}$...+$3^{51}$ ) - ( ${1}$+${3}$+$3^{2}$+...+$3^{50}$ )
${2A}$ = ${3}$+$3^{2}$+$3^{3}$...+$3^{51}$ - ${1}$-${3}$-$3^{2}$-...-$3^{50}$
${2A}$ = $3^{51}$ - ${1}$
${A}$ = $\frac{3^{51} - {1}}{2}$
$\text{Vậy A = $\frac{3^{51} - {1}}{2}$}$