1/2(cos2B-cos2A)
=1/2[2(cosB)^2-1-2(cosA)^2+1]
=(cosB)^2-(cosA)^2.(1)
sin(A+B)sin(A-B)
=(sinAcosB+cosAsinB)(sinAcosB-cosAsinB)
=(sinAcosB)^2-(cosAsinB)^2
=(cosB)^2[1-(cosA)^2]-(cosA)^2[1-(cosB)^2]
=(cosB)^2-(cosBcosA)^2-(cosA)^2+(cosAcosB)^2
=(cosB)^2-(cosA)^2.(2)
(1)=(2)
∴1/2(cos2B-cos2A)=sin(A+B)sin(A-B)
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