a) \((2 - \sqrt 2 )( - 5\sqrt 2 ) - {(3\sqrt 2 - 5)^2}\)
\( = - 10\sqrt 2 + 5\sqrt {{2^2}} - (18 - 30\sqrt 2 + 25)\)
\( = - 10\sqrt 2 + 10 - 18 + 30\sqrt 2 - 25 \)\(= 20\sqrt 2 - 33\)
b)
\( \displaystyle 2\sqrt {3a} - \sqrt {75a} + a\sqrt {{{13,5} \over {2a}}} \)\(\displaystyle - {2 \over 5}\sqrt {300{a^3}} \)
\( \displaystyle = 2\sqrt {3a} - \sqrt {25.3a} + a\sqrt {{{9.3} \over {4a}}} \)\(\displaystyle- {2 \over 5}\sqrt {100{a^2}.3a} \)
\( \displaystyle = 2\sqrt {3a} - 5\sqrt {3a} + {3 \over 2}\sqrt {3a} - 4a\sqrt {3a} \)
\( = \sqrt{3a} (2-5+\dfrac{3}{2}-4a) \)\(=-(1,5+4a) \sqrt {3a}\) (với \(a>0\))
