cos[(a+b)/2]=cos[(a-b/2)-(a/2-b)]
=cos(a-b/2)cos(a/2-b)+sin(a/2-b)sin(a-b/2)
cos(a-b/2)=-1/9,sin(a/2-b)=2/3
且π/2
sin(a-b/2)=根号[1-(1/9)^2]=(根号80)/9
cos(a-b/2)cos(a/2-b)+sin(a/2-b)sin(a-b/2)
=-1/9*(根号5)/3+(根号80)/9*2/3
=7(根号5)/27
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