公式cosαcosβ = 1/2*[cos(α+β)+cos(α-β)]
cos(1/2(a+b))cos(1/2(a-b))
=1/2*{cos(1/2(a+b)+1/2(a-b)]+cos[1/2(a+b)+1/2(a-b)]}
=1/2(cosa+cosb)