The rule for converting sin into cos and vice versa is: sin α = cos ( 90° - α ) or cos α = sin ( 90° - α ) In this case: cos ( 6 x ) = sin ( 90° - 6 x ) sin ( 90° - 6 x ) = sin ( 3 x - 9°) 90° - 6 x = 3 x - 9° 90° + 9° = 3 x + 6 x 99° = 9 x x = 99° : 9 = 11° Answer: C ) x = 11°
