Answer: a 2cos^2 (9x)
Step-by-step explanation:
Answer: A
1 + cos(18x) becomes
1 + cos(9x + 9x)
1 + cos²(9x) - sin²(9x)
1 + cos²(9x) - (1 - cos²(9x))
1 + cos²(9x) - 1 + cos²(9x)
2 cos²(9x)