\(\begin{aligned} \frac{d^{2}-c^{2}}{\left(c-d\right)^{2}} & =\frac{\left(d-c\right)\left(d+c\right)}{\left(c-d\right)^{2}} & & \text{laut 3. binomische Formel}\\ & =\frac{\left(d-c\right)\left(d+c\right)}{\left(d-c\right)^{2}} & & \text{wegen }a^{2}=(-a)^{2}\\ & =\frac{\left(d-c\right)\left(d+c\right)}{\left(d-c\right)\left(d-c\right)} & & \text{laut Definition }\square^{2}\\ & =\frac{d+c}{d-c} & & \text{wegen Kürzen mit }d-c \end{aligned}\)