F(x)=sin（π/2+x）cosx-sinxcos（π-x）=-cos^2 x+sinxcosx=1/2sin2x-1/2cos2x+1/2=(根号2)/2*sin(2x-π/4)+1/2周期=2π/2=πF(A)=1得到sin(2A-π/4)=(根号2)/2=sin(π/4)由于A是锐角，可知A=π/4由正弦定理AC/sinB=BC/sinAAC=根号6
