Electrical Engineering - DSP ogue signal question?

For the square wave, you need to know the attenuation curve of the filter (no of poles). A single-pole filter will have an attenuation of -6db/octave. The first harmonic of the square wave is at 12 kHz (n=3) and has an amplitude of 4/(3π) times the square wave amplitude. The next harmonic (n=5) is at 20 kHz and has an amplitude of 4/(5π). The first octave above the filter cutoff is 20 kHz, so the attenuation of the second harmonic is 6 db. The attenuation of the first harmonic is less than 3 db. The resulting wave will be smoothed somewhat because of the reduction in the amplitude of the higher harmonics.