$($a$)(30\%)$ Please find the Thevenin equivalent circuits between terminal a and $b$ under $=2000ra\frac{d}{s}$ and $=4000ra\frac{d}{s}.$

Hint: Consider whether to turn on or turn off the voltage source and current source based on the frequency you choose.

$($b$)(10\%)$ Please draw two phasor diagrams of the two Thevenin equivalent impedances under the two frequencies and $=4000ra\frac{d}{s}.$