2π＜a＜3π
π＜a/2＜3π/2
所以sin(a/2)<0,cos(a/2)<0
所以sin(a/2)+cos(a/2)<0
[sin(a/2)+cos(a/2)]²
=sin²(a/2)+cos²(a/2)+2sin(a/2)cos(a/2)
=1+sina
=4/3
所以sin(a/2)+cos(a/2)=-2√3/3