2. coa-cos⁴ a + sin⁴ a;

2. $$\cos^2 \alpha - \cos^4 \alpha + \sin^4 \alpha = \cos^2 \alpha - (\cos^4 \alpha - \sin^4 \alpha) = \cos^2 \alpha - (\cos^2 \alpha - \sin^2 \alpha)(\cos^2 \alpha + \sin^2 \alpha) = \cos^2 \alpha - (\cos^2 \alpha - \sin^2 \alpha) = \cos^2 \alpha - \cos^2 \alpha + \sin^2 \alpha = \sin^2 \alpha$$

Ответ: $$\sin^2 \alpha$$