a) \(\left( {4\sqrt x - 2\sqrt x } \right)\left( {\sqrt x - \sqrt {2x} } \right)\)
\( = 4\sqrt {{x^2}} - 4\sqrt {2{x^2}} - \sqrt {2{x^2}} + \sqrt {4{x^2}} \)
\( = 4x - 4x\sqrt 2 - x\sqrt 2 + 2x \)
\(= 6x - 5x\sqrt 2 \) (với \(x \ge 0\))
b) \(\left( {2\sqrt x + \sqrt y } \right)\left( {3\sqrt x - 2\sqrt y } \right)\)
\( = 6\sqrt {{x^2}} - 4\sqrt {xy} + 3\sqrt {xy} - 2\sqrt {{y^2}} \)
\( = 6x - \sqrt {xy} - 2y\) (với \(x \ge 0\), \(y \ge 0\))
