Power dissipation calculation across resistor from FM equation

Calculate the dissipation in power across 20Ω resistor for the FM signal
v(t)= 20 cos(6600t+ 10sin2100t)

Options:

a) 5W
b) 20W
c) 10W
d) 400W

Correct Answer: a) 5W

Explanation:

A standard FM signal is represented by
v(t)= A_c cos(2πf_ct+ k_fsin2πf_mt)
A_c = carrier amplitude
f_c= carrier frequency
k_f= modulation index
f_m = modulating frequency
k_f = frequency deviation/modulating frequency
the power dissipated across 20Ω resistor is given by
V_rms²/R
=(20/√2)²/R
= 5W