解：由 cos(A+B)=cosA*cosB-sinA*sinB得 cos(A/2+A/2)=cosA/2*cosA/2-sinA/2*sinA/2 cosA=[cosA/2]^2-[sinA/2]^2 cosA=[cosA/2]^2-(1-[cosA/2]^2) cosA=2[cosA/2]^2-1∴[cosA/2]^2=(1+cosA)/2
