An engineering question about an RC Lag circuit...

Question

An RC lag circuit has a critical frequency of 3.5 kHz. If the resistance value is 2 kΩ. What is the Xc when f = 4 kHz?

i am trying to check my answer. But i know i got it wrong.

Answer 1

if we accept "critical frequency" = 1 / 2piRC

then

C = 1 / (2pi *R * fcrit)

C = .159 / (2000 * 3500)

C = 22.7 * 10^-9

C = 22.7 nF

Xc at 4kHz =

1 / (2pi f C) =

.159 / (4000 * 22.7*10^-9)=

.159 / (90.8 * 10^-6) =

1.751 * 10^3 =

Xc = 1751 ohms at 4000 Hz  <===== my answer