\[\operatorname{dc}(u+v)=\frac{\operatorname{dc}u\operatorname{dc}v+{k^{\prime}}%
^{2}\operatorname{sc}u\operatorname{nc}u\operatorname{sc}v\operatorname{nc}v}{%
1-{k^{\prime}}^{2}{\operatorname{sc}}^{2}u{\operatorname{sc}}^{2}v},\]