124. Разложите на множители: 4) (a + b - c)² – (a – b + c)².

4) $$(a + b - c)^2 - (a - b + c)^2 = ((a + b - c) - (a - b + c))((a + b - c) + (a - b + c)) = (a + b - c - a + b - c)(a + b - c + a - b + c) = (2b - 2c)(2a) = 4a(b-c)$$