A condenser of 250µF is connected in parallel to a coil of inductance 0.16 mH, while its effective resistance is 20Ω. Determine the resonant frequency.

For the given case we have standard formula for resonant frequency as:
f=1/(2π√(LC−R²/4L²))
f=1/(2π√(0.16×10⁻³×250×10⁻⁶−(0.16×10⁻³)²))
On directly putting the given values in he equation we get:
ω=7.98×10⁵