Answer- cos(2A) - 1
*** I'm not very sure of the answer. Because I'm not very good at math.
How to make-
cos⁴A - cos²A = sin⁴A - sin²A
= cos(A)⁴ -1 - sin(A)⁴
= (cos(A)² - sin(A)²) × (cos (A)² + sin(A)²) -1
= cos (2A) × (cos(A)² + sin(A)²) -1
= cos(2A) × 1 - 1
= cos(2A) - 1
