OBJECTIVE: To creat a MATLAB AM-LC and FM modulated signals and then
to demodulate the signals.

Please help me with the code for the following :

Q.A.Create a 100 second time axis sampling at 100 hz (?t=0.01 second).
a. Create a 5 Hz sinusoid,y,using the time axis ,plot(t,y)?
b.Create a 20 hz carrier sinusoid,z,using the time axis .plot(t,z)?
c.On a point-by-point basis multilpy (1+0.5y)*z to form an AM/LC
signal with modulation index 0.5.Plot this.Zoom in and look at over
several cycles of the modulating signal.Plot this.
d.Using FFT 
a.compute the spectra for part c.Plot these in terms of
magnitude.Observe the presence of 20 Hz carrier.
b.Measure and compute the modulation index from the spectral amplitudes .

B.Coherent Demodulation.
   a.Multiply the modulated waveform by the vector containing the
carrier and low pass filter.Plot the output.
C.Envelope detection
a.Take modulated waveform and form abs(x).plot this.
b.Design low pass filters and filter the waveform until you recover
the orginal waveform.