R.H.*. = (cosA + cos3A) + (cos5A + cos7A)
= [cos(2A − A) + cos(2A + A)] + [cos(6A − A) + cos(6A + A)]
= 2cosA⋅cos2A + 2cosA⋅cos6A
= 2cosA[cos(4A −2A) + cos(4A +2A)]
= 2cosA⋅2cos2A⋅cos4A
=(2sinAcosA)/sinA . 2cosA.cos4A
=(2sin2A.cos2A).cos4A/sinA
=sin4A.cos4A/sinA = sin8 A/2 sinA
