Khai triển và rút gọn các biểu thức sau:

a) \({\left( {2 + \sqrt 2 } \right)^4}\);

b) \({\left( {2 + \sqrt 2 } \right)^4} + {\left( {2 - \sqrt 2 } \right)^4}\);

c) \({\left( {1 - \sqrt 3 } \right)^5}\).

a) \({\left( {2 + \sqrt 2 } \right)^4}\)= \(C_4^0{2^4} + C_4^1{.2^3}\left( {\sqrt 2 } \right) + C_4^2{.2^2}{\left( {\sqrt 2 } \right)^2} + C_4^3.2.{\left( {\sqrt 2 } \right)^3} + C_4^4.{\left( {\sqrt 2 } \right)^4}\)

= 16 + 32\(\sqrt 2 \) + 48 + 16\(\sqrt 2 \) + 4
\( = 68 + 48\sqrt 2 \).

Vậy \({\left( {2 + \sqrt 2 } \right)^4} = 68 + 48\sqrt 2 \).

b) Ta có: \({\left( {2 - \sqrt 2 } \right)^4}\)= \(C_4^0{2^4} + C_4^1{.2^3}\left( { - \sqrt 2 } \right) + C_4^2{.2^2}{\left( { - \sqrt 2 } \right)^2} + C_4^3.2.{\left( { - \sqrt 2 } \right)^3} + C_4^4.{\left( { - \sqrt 2 } \right)^4}\)

= 16 – 32\(\sqrt 2 \) + 48 – 16\(\sqrt 2 \) + 4.

\( = 68 - 48\sqrt 2 \).

Khi đó: \({\left( {2 + \sqrt 2 } \right)^4} + {\left( {2 - \sqrt 2 } \right)^4} = 68 + 48\sqrt 2 + 68 - 40\sqrt 2 = 136.\)

c) \({\left( {1 - \sqrt 3 } \right)^5}\).

\( = C_5^0{.1^5} + C_5^1{.1^4}\left( { - \sqrt 3 } \right) + C_5^2{.1^3}{\left( { - \sqrt 3 } \right)^2} + C_5^3{.1^2}.{\left( { - \sqrt 3 } \right)^3} + C_5^4{.1^1}{\left( { - \sqrt 3 } \right)^4} + C_5^5.{\left( { - \sqrt 3 } \right)^5}\)

\( = 1 - 5\sqrt 3 + 30 - 30\sqrt 3 + 45 - 9\sqrt 3 \)

\( = 76 - 44\sqrt 3 \).

Vậy \({\left( {1 - \sqrt 3 } \right)^5} = 76 - 44\sqrt 3 .\)