原式===> cos(2a-π/3)-[cos2a-cos(π/4)]=1/2===> cos2acos(π/3)+sin2asin(π/3)-coa2a+√2/2=1/2===> (1/2)cos2a+(√3/2)sin2a-cos2a=(1-√2)/2===> (√3/2)sin2a-(1/2)cos2a=(1-√2)/2稍等一下.===> -cos(2a+π/3)=(1-√2)/2
===> cos[π-(2a+π/3)]=(1-√2)/2
===> cos(2π/3-2a)=(1-√2)/2