I've re-written it exactly as it was handed to me, hence, being stumped.
We know nothing about the peak and average powers, Ki is, I assume the impedance angle.
Show that the resonance bandwidth corresponds to the frequency range for
which –1 < tan χ < +1. (The resonance bandwidth is the range for which the
average power is greater than 0.5 times the peak power.)
I'm pretty damn stumped with this.