b
(b)
$y_1=4 \sin 500 \pi t$
$y_2=2 \sin 506 \pi t$
Frequency of $y_1\left(f_1\right)=\frac{\omega}{2 \pi}=\frac{500 \pi}{2 \pi}=250 \,Hz$
Frequency of $y_2\left(f_2\right)=\frac{\omega}{2 \pi}=\frac{506 \pi}{2 \pi}=253 \,Hz$
Intensity relation $\frac{A_{\max }}{A_{\min }}=\frac{\left(A_1+A_2\right)^2}{\left(A_1-A_2\right)^2}$
$=\frac{36}{4}=9$