Упростите выражения (подробно): (a+3) (a-3) - 2a (4+a) (y-3) (5-y) - (4-y) (y+4)

(a+3)(a-3)-2a(4+a) = a×a-3×3-2a×4-2a×a = a²-9-8a-2a² = -a²-8a-9. (y-3)(5-y)-(4-y)(y+4) = y×5-y×y-3×5+3×y-(4-y)(4+y) = 5y-y²-15+3y-(4×4-y×y) = -y²+8y-15-16+y² = 8y-31.