Решение
4.
2/[(x-3)*(x+3)] - 1/[x*(x - 3)] - 4/[x*(x+3)] = 0
(2x - x - 3 - 4x + 12) / [(x*(x² - 9)] = 0
(9 - 3x) / [(x*(x² - 9)] = 0
9 - 3x = 0
x*(x² - 9) ≠ 0
3x = 9. x = 3
x≠0; x²≠9
x≠-3;x≠3
уравнение решений не имеет
5.
1) 2/(9 - 6y + y²) + 1/(9 - y²) = 2/(3-y)² + 1/[(3-y)*(3+y)] =
((6 + 2y + 3 - y) / [(3-y)² * (3 + y)] = (9 + y) / [(3-y)² * (3 + y)]
2) [(y-3)² * (9+y)] / [(3-y)² * (3 + y)] = (9 + y) / (3 + y)
3) 2y / (y + 3) + (9 + y) / (3 + y) = (2y + 9 + y) / (y + 3) = (3y + 9)/(y + 3) =
= [3*(y + 3)] / (y + 3) = 3 не зависит от значения у