Find the resonant frequency of an A.C circuit having a 40.0 Ω resistor, a 3.00 mH inductor, and a 5.00 μF capacitor.

At resonance, the inductive and capacitive reactances are equal. The frequency is f = 1 ÷ (2π√(LC)). The resistor does not affect it.

LC = 3.00 × 10⁻³ × 5.00 × 10⁻⁶ = 1.5 × 10⁻⁸.

√(LC) = 1.225 × 10⁻⁴. Then 2π × 1.225 × 10⁻⁴ = 7.70 × 10⁻⁴.

f = 1 ÷ 7.70 × 10⁻⁴ ≈ 1300 Hz = 1.3 kHz.