We previously computed the spectrum of the FM signal defined by 

x_c1 (t) = A_c cos[2πf_ct + β sin(2πf_mt)]

Now assume that the modulated signal is given by

x_c2 (t) = A_c cos[2πf_ct + β cos(2πf_mt)]

Show that the amplitude spectrum of x_c1 (t) and x_c2 (t) are identical. Compute the phase spectrum of x_c2 (t) and compare with the phase spectrum of x_c1(t).