Question

$cos { }^{4} \alpha - sin {}^{4} \alpha = cos {}^{2} \alpha$
​

Answer 1

Cos⁴α - Sin⁴α = (Cos²α)² - (Sin²α)² = (Cos²α - Sin²α)(Cos²α + Sin²α) =

Cos2α * 1 = Cos2α