\[\operatorname{cn}(u+v)=\frac{\operatorname{cn}u\operatorname{cn}v-%
\operatorname{sn}u\operatorname{dn}u\operatorname{sn}v\operatorname{dn}v}{1-k^%
{2}{\operatorname{sn}}^{2}u{\operatorname{sn}}^{2}v},\]