Simplify

$$\small{\text{ $ \dfrac{ \dfrac{c}{d}-\dfrac{d}{c} } { \dfrac{1}{c}-\dfrac{1}{d} } = \dfrac{ \dfrac{c^2-d^2}{dc} } { \dfrac{d-c}{dc} } = \left( \dfrac{c^2-d^2}{dc} \right) \cdot \left( \dfrac{dc}{d-c} \right) = \dfrac{c^2-d^2}{d-c} = -\dfrac{c^2-d^2}{c-d} = -\dfrac{(c-d)\cdot (c+d) }{c-d} = - (c+d) $}}\\\\$$

Thanks. That's the answer I also got, when I tried it the second time. :)