\[\operatorname{sd}(u+v)=\frac{\operatorname{sd}u\operatorname{cd}v\operatorname%
{nd}v+\operatorname{sd}v\operatorname{cd}u\operatorname{nd}u}{1+k^{2}{k^{%
\prime}}^{2}{\operatorname{sd}}^{2}u{\operatorname{sd}}^{2}v},\]