cos(A+B)*cos(A-B)=1/3
(1/2)*(cos(2A)+cos(2B))=1/3.(積化和差)
cos(2A)+cos(2B)=2/3
(cosA)^2-(sinB)^2
=(cos2A+1)/2+(cos2B-1)/2
=(cos2A+cos2B)/2=1/3
積化和差公式:
sinA*cosB=(1/2)*[sin(A+B)+sin(A-B)]
cosA*sinB=(1/2)*[sin(A+B)-sin(A-B)]
sinA*sinB=-(1/2)*[cos(A+B)-cos(A-B)]
cosA*cosB=(1/2)*[cos(A+B)+cos(A-B)]
和差化積公式也有四個公式(略):