[กขฃคฅฆง]

18. $a^3+a^2-27a-27 = 0$

$a^3+(1-\sqrt{3} )a^2-(36+\sqrt{3} )a-36 = 0$

$\sqrt{3} a^2+(9+\sqrt{3} )a+9 = 0$

$a = \frac{-9-\sqrt{3} \pm \sqrt{84-34\sqrt{3} } }{2\sqrt{3} }$