Chapter 4
Exercise 40:
Hint:
Note: V₀ is meant to indicate the voltage across the parallel branches in the schematic of the circuit.
When is the net reactance equal zero for the network comprised of the capacitor in parallel with the resistor/inductor combination?

Ans:
The resonant frequency will be that for which R² = X_L(X_C - X_L).
For the circuit parameters stated, this will be satisfied when f = 78 kHz.
[Please see detailed solution!]

Exercise 42:
Hint:
The net reactance for the network comprised of the capacitor in parallel with the resistor/inductor combination turns out to be purely resistive, 400 ohm.

Ans:
22 mA \/ 0 deg.

Chapter 6
Exercise 49:
Hint:
Consider the output as the voltage across the parallel RC circuit.

Find the resonant frequency by finding the frequency at which the magnitude of the function above is a maximum. It is not possible to zero out the reactance by changing the frequency. You may ignore the question about bandwidth...

Ans:
The resonant frequency is 4.4 rad/sec. [Updated - please see detailed solution!]