[tex]\begin{gathered} \frac{c^2-d^2}{d^2+cd-2c^2} \\ \frac{\left(c+d\right)\left(c-d\right)}{d^2+cd-2c^2} \\ \frac{\left(c+d\right)\left(c-d\right)}{\left(2c+d\right)\left(-c+d\right)} \\ -\frac{c+d}{2c+d} \end{gathered}[/tex]
answer:
[tex]\frac{-c-d}{d+2c}[/tex]
the third one
