O17-C037
> restart;
> u:=n->cos(n*x)/ln(n);

                                    cos(n x)
                          u := n -> --------
                                     ln(n)

> s:=n->sum(u(k),k=2..n);

                                     n
                                   -----
                                    \
                         s := n ->   )   u(k)
                                    /
                                   -----
                                   k = 2

> plot([s(50),s(60),s(70)],x=0..2*Pi,color=[blue,red,green]);

>