原式={1+cos[2(x/2-7π/8)]}/2-{1+cos[2(x/2+7π/8)]}/2=[1+cos(x-7π/4)]/2-[1+cos(x+7π/4)]/2=1/2[cos(x-7π/4)-cos(x+7π/4)]=1/2[(cosxcos7π/4+sinxsin7π/4)-(cosxcos7π/4-sinxsin7π/4)]=sinxsin7π/4=-√2(sinx)/2
