cosπ/12cos5π/12
=cos（π/2-5π/12）cos5π/12
=(cosπ/2cos5π/12+sinπ/2sin5π/12)cos5π/12
=sin5π/12cos5π/12
=1/2sin5π/6
=1/4
积化和差
cosπ/12cos5π/12＝2cos（π/12+5π/12）cos（π/12-5π/12）
=2cosπ/2cosπ/3
=0*cosπ/3
=0
cos π /12cos5π /12
=cos（π/2-5π/12）cos5π/12
=sin5π/12 cos5π/12
=1/2sin5π/ 6
=1/4
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