(5х-9)^2-(5х+9)^2= (5x-9)(5x-9)-(5x+9)(5x+9) = (25x2-45x-45x+81)-(5x+9)(5x+9) = 1(25x2-45x-45x+81)+(-25x2-45x-45x-81) = 25x2-45x-45x+81+(-25x2-45x-45x-81) = 25x2-45x-45x+81-25x2-45x-45x-81 = – 180x
(5х-9)^2-(5х+9)^2= (5x-9)(5x-9)-(5x+9)(5x+9) = (25x2-45x-45x+81)-(5x+9)(5x+9) = 1(25x2-45x-45x+81)+(-25x2-45x-45x-81) = 25x2-45x-45x+81+(-25x2-45x-45x-81) = 25x2-45x-45x+81-25x2-45x-45x-81 = – 180x