Applicare la formula: $\frac{a}{b}\frac{c}{f}$$=\frac{ac}{bf}$, dove $a=20m^2n-mn$, $b=9x^2-30xy+25y^2$, $c=n^2+n$, $a/b=\frac{20m^2n-mn}{9x^2-30xy+25y^2}$, $f=m^2-n^2$, $c/f=\frac{n^2+n}{m^2-n^2}$ e $a/bc/f=\frac{20m^2n-mn}{9x^2-30xy+25y^2}\frac{n^2+n}{m^2-n^2}$