Ta có :
$\frac{m}{n}$ = $\frac{p}{q}$ ⇒ $\frac{mp}{nq}$ = $\frac{$p^{2}$ }{$q^{2}$ }$
$\frac{m}{n}$ = $\frac{p}{q}$ ⇒ $\frac{$m^{2}$ }{$n^{2}$ }$ = $\frac{mp}{nq}$
⇔ $\frac{mp}{nq}$ = $\frac{$p^{2}$ }{$q^{2}$ }$ = $\frac{$m^{2}$ }{$n^{2}$ }$
$\alpha$ =$\frac{$p^{2}$+ $m^{2}$ }{$q^{2}$+ $n^{2}$}$