x+y=x.y=3 berapakah x^3 +y^3

(x + y)^3
= (x^2 + 2xy + y^2)(x + y)
= x^3 + x^2y + 2x^2y + 2xy^2 + xy^2 + y^3
= x^3 + 3x^2y + 3xy^2 + y^3

x^3 + y^3
= (x + y)^3 - (3x^2y + 3x^2y)
= (3)^3 - 3(x^2y + xy^2)
= 27 - 3(xy . x + xy . y)
= 27 - 3(3x + 3y)
= 27 - 3 . 3(x + y)
= 27 - 9(3)
= 27 - 27
= 0