Question

Se (x-y)²-(x+y)²=-20, entao x•y e igual a?

Answer 1

$(x-y)^2-(x+y)^2=-20\\ \\ x^2-2xy+y^2-(x^2+2xy+y^2)=-20\\ \\ x^2-2xy+y^2-x^2-2xy-y^2=-20\\ \\ -4xy=-20\\ \\ \boxed{xy=5}$