Đáp án:
$\begin{array}{l}
a)M = {x^2} + {y^2} = {\left( {x + y} \right)^2} - 2xy\\
= {\left( { - 9} \right)^2} - 2.18 = 81 - 36 = 45\\
b)N = {x^4} + {y^4}\\
= {\left( {{x^2} + {y^2}} \right)^2} - 2{x^2}{y^2}\\
= {M^2} - 2.{\left( {xy} \right)^2}\\
= {45^2} - {2.18^2}\\
= 1377\\
c)P = {x^2} - {y^2}\\
= \left( {x + y} \right)\left( {x - y} \right)\\
= - 9.\left( {x - y} \right)\\
= - 9.\sqrt {{{\left( {x - y} \right)}^2}} \\
= - 9.\sqrt {{{\left( {x + y} \right)}^2} - 4xy} \\
= - 9.\sqrt {{{\left( { - 9} \right)}^2} - 4.18} \\
= - 27
\end{array}$
