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\item[(1)]$\cos5\theta=f(\cos\theta)$をみたす多項式$f(x)$を求めよ．
\item[(2)]$\displaystyle\cos\frac{\pi}{10}\cos\frac{3\pi}{10}\cos\frac{7\pi}{10}\cos\frac{9\pi}{10}=\frac{5}{16}$
を示せ．
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