Applicare l'identità trigonometrica: $\sin\left(a\right)\sin\left(b\right)$$=\frac{\cos\left(a-b\right)-\cos\left(a+b\right)}{2}$, dove $a=\frac{\pi nx}{l}$ e $b=\frac{\pi mx}{l}$

Applicare la formula: $-\frac{b}{c}$$=\frac{expand\left(-b\right)}{c}$, dove $b=\pi mx$ e $c=l$

Applicare la formula: $\frac{a}{b}+\frac{c}{b}$$=\frac{a+c}{b}$, dove $a=\pi nx$, $b=l$ e $c=-\pi mx$

Applicare la formula: $\frac{a}{b}+\frac{c}{b}$$=\frac{a+c}{b}$, dove $a=\pi nx$, $b=l$ e $c=\pi mx$