Ta có :
$\begin{array}{l}
a = 2 + {2^2} + {2^3} + {2^4} + {2^5} + ... + {2^{30}}\\
\Rightarrow 2.a = 2.\left( {2 + {2^2} + {2^3} + {2^4} + {2^5} + ... + {2^{30}}} \right)\\
\Rightarrow 2.a = {2^2} + {2^3} + {2^4} + {2^5} + {2^6} + ... + {2^{31}}\\
\Rightarrow 2.a - a = {2^2} + {2^3} + {2^4} + {2^5} + {2^6} + ... + {2^{31}}\\
\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\, - \left( {2 + {2^2} + {2^3} + {2^4} + {2^5} + ... + {2^{30}}} \right)\\
\Rightarrow a = {2^2} + {2^3} + {2^4} + {2^5} + {2^6} + ... + {2^{31}} - 2 - {2^2} - {2^3} - {2^4} - {2^5} - ... - {2^{30}}\\
\Rightarrow a = {2^{31}} - 2\\
\Rightarrow a + 2 = {2^{31}} - 2 + 2\\
\Rightarrow a + 2 = {2^{31}}
\end{array}$
