MATLAB: How do i plot

plotting sinusoidal time signal

Im having problem plotting the sinusoidal time signal for (5+cos(20pt))cos(2p(100)t), having Dt=0.0001s, t=0~0.02 sec.

Is this what you meant?

dt = 0.0001;
t  = 0:dt:0.02;
y  = (5+cos(20*pi*t)).*cos(2*pi*100*t); 
figure;
plot(t,y);

Output:

figure out what azzi was doing in the math which would lead you to figure out what to change.

t=0:0.01:24
y=sin(t*pi).*(t<15) + 3*sin(t*pi).*(t>=15)
plot(t,y)

Use

Nt = 1000
t  = linspace(0,100,Nt+1)
d  = 0.005*sin(0.5*pi*t).*cos(0.5*pi*t)
plot(t,d)

Any reason you are using a 3-dimensional array d ?