来自鲍晓慧的问题
(cosπ/12+sin11π/12)(cosπ/12+sin13π/12)
(cosπ/12+sin11π/12)(cosπ/12+sin13π/12)
(cosπ/12+sin11π/12)(cosπ/12+sin13π/12)
=(cosπ/12+sin1π/12)(cosπ/12-sin1π/12)
=（cosπ/12）^2-(sin1π/12)^2
=cosπ/6
=√3/2
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