sin(π/12)-sin(5π/12)+2sin(π/8)sin(3π/8)
=sin(π/12)-cos（π/12)+2sin(π/8)cos(π/8)
=√2sin（π/12－π/4）+sin（π/4）
=-√2sin（π/6）+sin（π/4）
=-√2/2＋√2/2
=0