cosπ/8的4次方- sinπ/8的4次方
=（cos²π/8+sin²π/8）（cos²π/8-sin²π/8）
=cos²π/8-sin²π/8=cosπ/4
=√2/2
sinπ/12-√3cosπ/12
=2（1/2sinπ/12-√3/2cosπ/12）
=2（cosπ/3sinπ/12-sinπ/3cosπ/12）
=2sin（π/12-π/3）
=-2sin（π/4）
=-√2