Answer:
Step-by-step explanation:
FYI,
a² - b² = (a + b)(a - b)
f(x) = [tex]cos^{4} x[/tex] - [tex]sin^{4} x[/tex] = (cos²x + sin²x)(cos²x - sin²x) =
= (cos²x + sin²x)(cos x + sin x)(cos x - sin x)
If you need to simplify, then
f(x) = 1 × (cos²x - sin²x) = cos(2x)
