[lnพwsะบุ๑sสุ๑xล่o]

11.

$Px^2-Qx-2=0$

$a+b=\frac{Q}{P} $

$ab=\frac{-2}{P} $

$\frac{1}{a^2} +\frac{1}{b^2} =\frac{a^2+b^2}{(ab)^2}$
$=\frac{(a+b)^2-2ab}{(ab)^2} =\frac{\frac{Q}{P^2} ^2+\frac{4}{P} }{\frac{4}{P^2} } =\frac{Q^2+4P}{4} $