MAXIMA does not plot a wave packet

[Rajib & Susmita Bandopadhyay]

Dear friends,
We all know that when two monochromatic waves with slightly varying
frequencies meet we get a wave packet with a principal maxima at a point
under certain conditions.
I wrote the following lines in maxima to write an equation for a
wave-packet and generate a plot for it.
I wrote the following inputs:
/* [wxMaxima: input   start ] */
F_1 (x):= sum(Cos(x+0.09*n*x),n,-10,10);
F_1 (x);
/* makelist(Cos(x+0.09*n*x),n,-10,10);*/
/* plot2d(( makelist(Cos(x
+0.09*n*x),n,-10,10) ),[x,-50,50],[y,-25,25],[nticks,1000])$ */
plot2d((F_1(x))^2,[x,-50,50],[nticks,1000])$
/* [wxMaxima: input   end   ] */

The output lines are as attached:
(%o2) F_1(x):=sum(Cos(x+0.09*n*x),n,-10,10)
(%o3)
Cos(1.9*x)+Cos(1.81*x)+Cos(1.72*x)+Cos(1.63*x)+Cos(1.54*x)+Cos(1.45*x)+Cos(1.36*x)+
Cos(1.27*x)+Cos(1.18*x)+Cos(1.09*x)+Cos(x)+Cos(0.91*x)+Cos(0.82*x)+Cos(0.73*x)+Cos(0.64*x)
+Cos(0.55*x)+Cos(0.46*x)+Cos(0.37*x)+Cos(0.28*x)+Cos(0.19*x)+Cos(0.1*x)
plot2d: expression evaluates to non-numeric value somewhere in plotting
range.
Where am I going wrong, why maxima is not plotting the packet?
                Maxima version: In Debian Squeeze 6.0.3
                wxWidgets: 2.8.10
                Unicode support: yes
                Maxima version: 5.21.1 
                Lisp: GNU Common Lisp (GCL) GCL 2.6.7 (a.k.a. GCL)
Regards

summation.jpeg
Description: JPEG image