I see a few problems:
Your f variable is still based on the original vector of t. You need to run:
f = 53*cos(104*pi*t).*exp(-1.35*t);
again, after you set your new t
Also, you have set t to be the same vector (ie, t = 0.0 : 0.01 : 5; ). I don't think you wanted this.
Finally, in the second figure you are calling:
plot(t,f(t));
but t is still a vector of times, not a vector of indices.
Perhaps you meant the following:
% First figure
t = 0.0 : 0.01 : 5.0;
f = 53*cos(104*pi*t).*exp(-1.35*t);
figure
plot(t,f);
xlabel('t(sec)');
ylabel('f(t)');
% Second figure, using only first 1/5th of t, f
idxs = 1:round(length(t)/5);
figure
plot(t(idxs),f(idxs));
xlabel('t(sec)');
ylabel('f(t)');
By the way, have you tried instead just:
set(gca,'xlim',[0 1])
You can run this after the first plot, and it will "zoom in" on only the first portion from 0 to 1 along the x axis. The rest will be excluded from view.
