題組內容

六.(13%) Fourier series says that any periodic function can be expressed as a linear combination of infinite harmonic cosine and sine functions.

(二) (5%) Find a vector g(x) in a subspace spanned by only{1, cos(x), cos(2x), sin(x), sin(2x)} so that g(x) has the shortest distance to the function f(x) = x in the interval [-π,π].
 Note: In this problem, you may need: 
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