We show that the family of functions {sin(nx): n = 1,2,...} and {cos(nx): n = 0,1,2,...} are orthogonal on the interval [-pi, pi] (equivalently on the unit circle). We use the orthogonality to find a formula for the Fourier sine and Fourier cosine series coefficients. We use Euler's formula to relate these coefficients to the complex Fourier coefficients.
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