A) (x+y)² - 2xy = (x^2 + 2 . x. y + y^2) - 2xy = x^2 + 2xy + y^2 - 2xy = x^2 + y^2

  b) (5-2z)² - 25 = 10z
        [ 5^2 - 2.5 . 2z + (2z)^2] - 25 -10z = 0
                        25 - 10z + 4z^2 - 10z = 0
                            4z^2 - 20z +25 = 0