a) $\dfrac{1,35}{1,25}=\dfrac{0,1x}{0,2}$
$0,1x.1,25=1,35.0,2=0,27$
$(0,1.1,25).x=0,27$
$0,125.x=0,27$
$→x=0,27:0,125=2,16$
b) $\dfrac{2x}{9}=\dfrac{8}{x}$
$2x.x=8.9=72$
$2x^2=72$
$x^2=72:2=36$
\(→\left[ \begin{array}{l}x=\sqrt{36}=6\\x=-\sqrt{36}=-6\end{array} \right.\)