$M = \sqrt {3x - 2\sqrt {3x - 1} } - \sqrt {3x + 8 - 2\sqrt {3x - 1} } $
$M = \sqrt {{{(\sqrt {3x - 1} )}^2} - 2\sqrt {3x - 1} + 1} - \sqrt {{{(\sqrt {3x - 1} )}^2} - 2\sqrt {3x - 1} + 9} $
$M = \sqrt {{{(\sqrt {3x - 1} - 1)}^2}} - \sqrt {{{(\sqrt {3x - 1} - 3)}^2}} $
$M = \left| {\sqrt {3x - 1} - 1} \right| - \left| {\sqrt {3x - 1} - 3} \right|$
Nếu:
+ $x \geqslant \dfrac{1}{3}$ thì $M = \sqrt {3x - 1} - 1 - \sqrt {3x - 1} + 3 = 2$
+ $x < \dfrac{1}{3}$ thì $M = 1 - \sqrt {3x - 1} - 3 + \sqrt {3x - 1} = - 2$