Question

se ( x - y ) . ( x - y)- ( x+ y ) . ( x + y ) = -20 entao x . y e igual a ???

Answer 1

(x - y)(x - y) - ( x + y)(x + y) = - 20
x² - xy - xy + y² - ( x² + xy + xy + y²) = - 20
x² - 2xy + y² - ( x² + 2xy) + y² ) = - 20
x² - 2xy + y² - x² - 2xy - y² = - 20
x² - x² - 2xy - 2xy + y² - y² = - 20
               - 4xy = - 20 .(-1)
                 4xy = 20
                   xy = 20/4
                   xy = 5