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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^2 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^2 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^2 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."