Simplifying x^{4} + -1y^{4} Factor a difference between two squares. (x^{2} + y^{2})(x^{2} + -1y^{2}) Factor a difference between two squares. (x^{2} + y^{2})((x + y)(x + -1y))Final result:(x^{2} + y^{2})(x + y)(x + -1y)