When deriving Fourier series, an important step is to establish that the integral of the product of two periodic sin functions is 0 if they have a different frequency.
This then allows you to define Fourier series as the basis for an inner product space.
The usual derivation makes use of trig identities. Here's an example: Orthogonality of sine and cosine integrals.
However I don't find these derivations very intuitive.
Is there another way to see why the integral of sin/cos functions of different frequencies is always 0?