Đáp án:
$\begin{array}{l}
+ )x + y\\
= \sqrt {{{\left( {x + y} \right)}^2}} \\
= \sqrt {{x^2} + 2xy + {y^2}} \\
= \sqrt {18 + 2.5} \\
= \sqrt {28} \\
= 2\sqrt 7 \\
+ ){x^3} + {y^3}\\
= \left( {x + y} \right)\left( {{x^2} - xy + {y^2}} \right)\\
= 2\sqrt 7 .\left( {18 - 5} \right)\\
= 2\sqrt 7 .13\\
= 26\sqrt 7 \\
+ ){x^4} + {y^4}\\
= {x^4} + 2{x^2}{y^2} + {y^4} - 2{x^2}{y^2}\\
= {\left( {{x^2} + {y^2}} \right)^2} - 2.{\left( {xy} \right)^2}\\
= {18^2} - {2.5^2}\\
= 274
\end{array}$
