Hi,
You mean it will be interesting to see the teacher TRY to prove it <chuckle>
Unless he wants to call the infinite frequency zero phase because the output goes to zero, but i dont buy that either.
Carel:
If he still insists, ask him to give the correct frequency off by say plus or minus 20 percent without giving the true 'correct' frequency, this way we can test it and show that it is not correct for any frequency within that plus or minus 20 percent band. This way he doesnt have to 'give away' the right answer.
For example, if he says "1kHz" and that can be off by plus or minus 20 percent, then we can test all the frequencies between 800Hz and 1200Hz and show that none of those meet the criterion.