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