cos(x-y/2)=-1/9,sin(x/2-y)=2/3,0
因为cos(x-y/2)=-1/9 0
sin(x-y/2)=4*√5/9
因为sin(x/2-y)=2/3,0
cos(x/2-y)=√5/3
cos[（x-y/2）-(x/2-y)]=cos（x-y/2）*cos(x/2-y)+sin（x-y/2）*sin(x/2-y)=-1/9*√5/3+4*√5/9*2/3=7*√5/27
cos(x+y)=cos2[（x-y/2）-(x/2-y)]=2cos^2[（x-y/2）-(x/2-y)]-1=-239/729
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