The document is about Fourier series and solving problems using Fourier series. It discusses the trigonometric form of Fourier series, calculating Fourier coefficients, and using Fourier series to solve a sample problem about finding the power lost when passing a sawtooth wave through a communication channel that filters out frequencies above 5.5 Hz. The key steps are representing the sawtooth wave as a Fourier series, calculating the Fourier coefficients, and then determining the power in the frequencies below 5.5 Hz to find the power transmitted by the channel.