原式 = 根号2 * (根号2/2 * cosπ/12—根号2/2 * sinπ/12 )
= 根号2 * (sinπ/4cosπ/12—cosπ/4sinπ/12)
=根号2 * sin(π/4-π/12)
=根号2 * sin(π/6)
=根号2/2