The maximum impedance in an LCR circuit occurs at the resonant frequency. To find it, you need to calculate the impedance of the circuit components: a resistor (R), an inductor (L), and a capacitor (C). The impedance of the circuit is given by the formula:
\[ Z = \sqrt{R^2 + (X_L - X_C)^2} \]
where \( X_L = \omega L \) (inductive reactance) and \( X_C = \frac{1}{\omega C} \) (capacitive reactance).
The impedance is maximized when the difference between \( X_L \) and \( X_C \) is greatest, which happens when the circuit is far from resonance, or when \( X_L \neq X_C \). At resonance, the impedance is simply \( R \) because \( X_L = X_C \) and their effects cancel each other out.