31)
(3a - 7b)² - (-7a + 3b)² = 9a² - 42ab + 49b² - 49a² + 42ab - 9b² = -40a² + 40b² = -40(a² - b²) = -40(a - b)(a + b) = ( при a=2.8; b=2.2) = -40(2,8-2,2)(2,8+2,2) = -40*0,6*5 = 120
33)
9a - 15b = 3(3a - 5b)
3m + mn = m(3 + n)
5ab - 5ac = 5a(b - c)
6a² - 24ab = 6a(a - 4b)
x⁵ + x² = x²(x³ + 1)
28a²b - 7ab = 7ab(4a - 1)
3a² - 12a⁴ + 9a⁶ = 3a²( 1 - 4a² + 3a⁴)
10x⁴y² + 25x²y - 5x²y³ = 5xy(2x³y + 5x - xy²)