а²b - a - ab² + b - 2ab + 2 =
= a²b - ab² - 2ab - a + b + 2 == ab(a - b) - 1(a - b) + 2 == (ab - 1)(a - b) + 2.
а²b - a - ab² + b - 2ab + 2 =
= a²b - ab² - 2ab - a + b + 2 =
= ab(a - b) - 1(a - b) + 2 =
= (ab - 1)(a - b) + 2.