应该还有个条件是 x ≥ y2x² - xy - 5x + y + 4= x² + x² - xy - 4x - x + y + 4= x² - 4x + 4 + x(x-y) - (x - y)= (x - 2)² + (x - y)(x - 1) = 0(x - 2)² ≥ 0又因为 x ≥ y,x > 1...