（1）sin(5π/12)
=sin（2π/12+3π/12）
=sin(π/6+π/4)
=sin(π/6)cos(π/4)+cos(π/6)sin(π/4)
=（√2+√6)/4
（2）cos（-31π/12）
=cos（31π/12）
=cos（7π/12）
=sin（π/2-7π/12)
=-sin（π/12）
=-sin(π/3-π/4)
= -sin(π/3 )cos(π/4)+ cos(π/3)sin(π/4)
= -√3/2 * √2/2 + 1/2 *√2/2
= （-√6+√2)/4
（3）sin（-11π/12）
=- sin （11π/12）
= -sin(π-1/12π)
=-sin(π/12)
=-sin(π/3-π/4)
= -sin(π/3 )cos(π/4)+ cos(π/3)sin(π/4)
= -√3/2 * √2/2 + 1/2 *√2/2
= （-√6+√2)/4