如果是复指数形式的傅里叶级数，因为复指数函数的功率等于其系数的模的平方，直接把傅里叶系数平方求和就行；如果是三角形式的傅里叶级数，因为三角函数的功率等于其系数的模的平方的一半，需要把各次谐波傅里叶系数平方求和的一半 与直流分量的平方相加。-If the complex exponential form of Fourier series, complex exponential function because the power is equal to the square of the modulus of its coefficient, the square sum of Fourier coefficients directly on the line　If the triangular form of Fourier series, because the triangle function of power equal to the square of the modulus of its coefficient of half of the need to square the harmonic Fourier coefficients of half the sum of the square sum of the DC component.