Hi Mir Khadim,
The cutoff frequency is defined as the frequency where the power gain is a half, also called as the −3 dB-point, because
In the particular case of the moving average filter, just solve the following equation:
,where 
is the cut-off frequency. This should give you the following expression:
is the cut-off frequency. This should give you the following expression:
,which is an equation that has no analytical solution, then numericla methods should be used to find 
. Solutions at 
are not valid cause the transfer function is not defined. Then, the solutions should be found in the region 
.
. Solutions at are not valid cause the transfer function is not defined. Then, the solutions should be found in the region .Hope thhis can help to solve your question.
