Section 1.1 #1(d)

> g:=n->(1+(-1)^n)/(n^2-1)*cos(n*x);

> f:=N->2/Pi-2/Pi*sum(g(n),n=2..N);

> plot(f(5),x=-2*Pi..3*Pi);

> plot(f(10),x=-2*Pi..3*Pi);

> plot(f(100),x=-2*Pi..3*Pi);