因为π/2所以-π/2<-y<0-π/4<-y/2<0,π/4所以π/4根据题目中所给的数，可以所小范围π/2所以sin(x-y/2)=4根号5/9 cos(x/2-y)=根号5/3所以cos[(x+y)/2]=cos[(x-y/2)-(x/2-y)]=cos(x-y/2)cos(x/2-y)+sin(x-y/2)sin(x/2-y)=7根号5/27
