Answer:
9/2 [ cos(15x) + cos(3x) ]
Step-by-step explanation:
9cos(6x) . cos(9x) can be expressed as
= 9/2 * 2cos (6x) * cos(9x) next here we will apply the rule
2cosA . cosB = cos(A+B) + cos( A-B)
where A = 6x , B = 9x
hence : 9cos(6x) . cos(9x) expressed as a sum will be
= 9/2 * [cos( 6x+ 9x) + cos( 6x - 9x ) ]
= 9/2 [ cos (15x ) + cos(-3x) ]
the final sum = 9/2 [ cos(15x) + cos(3x) ]
