also please read this excerpt of what i wrote you can thank me later for it but i'll tell you parseval's identity it's more conceptual to think of l the space of measurable square integrable functions on pi pi as a complex inner product space hilbert space the idea is that the space has an orthonormal basis general orthornormal basis can be found by wavelets such as e in for n an integer and if f is a function in l you can take its inner product against each of these functions and get components of f the point is as you'd expect in any hilbert space the components are the only relevant data when it comes to taking inner products so you can find the inner product an integral of two functions just by knowing their fourier coefficients pretty cool math dude so basically i advise you to find the fourier coefficients of the function x on pi pi and take the inner product of this function with itself and apply parseval the proof of parseval is probably well beyond the scope of your education you'll see it when you get to university it involves proving the completeness of the trigonometric system in the l case which can be done nicely with convolution but i'll save the details to your lecturers at first year undergrad uni i notice you said that you didn't know what fourier series is and it shows since if you did know you'd see that the above is perfectly correct math feel free to consult anyone but don't remove perfectly legitimate math just because you don't understand it i don't want to insult you but i find you're insulting me by deleting text because you don't understand it