for n=1:50
    an=(-8*sin(n*pi/3)/(n*pi)-6*(cos(n*pi)-cos(n*pi/3))/(n*n*pi*pi)).*(abs(t)<3);
    X2= X2+an.*cos(n*pi*t/3);

    blad = (x(1,t)-X2(1,t)).^2;
end

mse = sqrt(blad)/length(t);