t=0:0.01:4;
f=-0.5*(t-2).*(u(t)-u(t-2))+0.5*(t-2).*(u(t-2)-u(t-4));
c=cos(5*pi*t);
f1=f.*c;
subplot(3,1,1)
plot(t,f);grid on;title('The baseband signal g(t)')
subplot(3,1,2)
plot(t,c);grid on;ylabel('Carrier')
subplot(3,1,3)
plot(t,f1);grid on;ylabel('Modulated carrier of AM')