Time Period of one Cycle is way too large
The equation xn=cos(2*pi*(F1)*n*T) expects n to be a sample index vector (n=0:N) where N is the total number of samples.  n*T or n/Fs produces the time vector.

Here is a way to plot a single cycle:

F1 = 100e3;
Fs = 1e6;
t = 0:(1/Fs):1/F1;  %Creates time vector for a single cycle
x = cos(2*pi*F1*t);
stem(t, x);


Note that 1/F1 gives the time period of a single cycle.