For any angle α, cos ^ 4 α - Sin ^ 4 α = (COS ^ 2 α + sin ^ 2 α) (COS ^ 2 α - Sin ^ 2 α) = cos ^ 2 α - Sin ^ 2 α = Cos2 α?
Square difference formula and Sin & # 178; a + cos & # 178; a = 1
Square difference formula and Sin & # 178; a + cos & # 178; a = 1