Fourier Series (finite case)

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Suppose we have a function $f \in C([-\pi,\pi])$ and another function $g(t)=\sum_{k=1}^{n}a_k \sin (kt)$. Determine the numbers $a_1, a_2,..., a_n$ for which the integral $\int_{-\pi}^{\pi}(f(t)-g(t))^2dt$ is minimal.