sin（π／4 -α）＝cos[π／2－（π／4 -α）]＝cos(π／4 +α）sin2(π／4 +α）＝2sin(π／4 +α）*sin（π／4 -α）＝1/3=cos2αα∈（π／2,π）,2α∈（π,2π）所以sin2α＜0sin2α=-(2根号2)/3sin4α＝－(4根号2)/9
