Answer:
see explanation
Step-by-step explanation:
Consider the left side
[tex]cos^{4}[/tex]A + [tex]sin^{4}[/tex]A - 2cos²Asin²A ← is the expansion of
= (cos²A - sin²A)²
Substitute sin²A = 1 - cos²A
= (cos²A - (1 - cos²A) )²
= (cos²A - 1 + cos²A)²
= (2cos²A - 1 )²
= right side , then proven
