Hi, This is not a homework, just a problem that I can't figure out. Please help. Assuming that I have a low pass filter with cutting frequency, fc and a signal with many different frequencies (with some below fc and some above fc) is applied to the input of the filter. As we know, the high-frequency components will be attenuated. My question: does energy of the signal lose after going through the filter? How about the the filter is parallel LC? In this case, with the assumption that L, C are ideal (no energy dissipated in these components) then the energy of the signal at input and output of LC filter has to be the same. However, some high-frequency components of the signal are destroyed now. How is that (energy at input and output are equal) possible?