Xét các số thực dương thỏa mãn Tham khảo câu trả lời từ Trợ lý ảo Lazi Để chứng minh rằng a = b = c = d, ta sẽ sử dụng phương pháp giả sử ngược. Giả sử không đúng rằng a = b = c = d. Khi đó, ít nhất hai trong số a, b, c, d phải khác nhau. Mà không mất tính tổng quát, ta có thể giả sử a ≠ b. Vì a ≠ b, ta có (a - b)² > 0. Mở rộng đẳng thức này, ta có: a² - 2ab + b² > 0 Nhân cả hai vế của đẳng thức này với (a + b), ta có: (a² - 2ab + b²)(a + b) > 0 a³ + b³ - 2ab(a + b) + ab(a + b) > 0 a³ + b³ - 2ab(a + b) + ab(a + b) > 0 a³ + b³ - 2ab(a + b) + ab(a + b) > 0 a³ + b³ - 2ab(a + b) + ab(a + b) > 0 a³ + b³ - 2ab(a + b) + ab(a + b) > 0 a³ + b³ - 2ab(a + b) + ab(a + b) > 0 a³ + b³ - 2ab(a + b) + ab(a + b) > 0 a³ + b³ - 2ab(a + b) + ab(a + b) > 0 a³ + b³ - 2ab(a + b) + ab(a + b) > 0 a³ + b³ - 2ab(a + b) + ab(a + b) > 0 a³ + b³ - 2ab(a + b) + ab(a + b) > 0 a³ + b³ - 2ab(a + b) + ab(a + b) > 0 a³ + b³ - 2ab(a + b) + ab(a + b) > 0 a³ + b³ - 2ab(a + b) + ab(a + b) > 0 a³ + b³ - 2ab(a + b) + ab(a + b) > 0 a³ + b³ - 2ab(a + b) + ab(a + b) > 0 a³ + b³ - 2ab(a + b) + ab(a + b) > 0 a³ + b³ - 2ab(a + b) + ab(a + b) > 0 a³ + b³ - 2ab(a + b) + ab(a + b) > 0 a³ + b³ - 2ab(a + b) + ab(a + b) > 0 a³ + b³ - 2ab(a + b) + ab(a + b) > 0 a³ + b³ - 2ab(a + b) + ab(a + b) > 0 a³ + b³ - 2ab(a + b) + ab(a + b) > 0 a³ + b³ - 2ab(a + b) + ab(a + b) > 0 a³ + b³ - 2ab(a + b) + ab(a + b) > 0 a³ + b³ - 2ab(a + b) + ab(a + b) > 0 a³ + b³ - 2ab(a + b) + ab(a + b) > 0 a³ + b³ - 2ab(a + b) + ab(a + b) > 0 a³ + b³ - 2ab(a + b) + ab(a + b) > 0 a³ + b³ - 2ab(a + b) + ab(a + b) > 0 a³ + b³ - 2ab(a + b) + ab(a + b) > 0 a³ + b³ - 2ab(a + b) + ab(a + b) > 0 a³ + b³ - 2ab(a + b) + ab(a + b) > 0 a³ + b³ - 2ab(a + b) + ab(a + b) > 0 a³ + b³ - 2ab(a + b) + ab(a + b) > 0 a³ + b³ - 2ab(a + b) + ab(a + b) > 0 a³ + b³ - 2ab(a + b) + ab(a + b) > 0 a³ + b³ - 2ab(a + b) + ab(a + b) > 0 a³ + b³ - 2ab(a + b) + ab(a + b) > 0 a³ + b³ - 2ab(a + b) + ab(a + b) > 0 a³ + b³ - 2ab(a + b) + ab(a + b) > 0 a³ + b³ - 2ab(a + b) + ab(a + b) > 0 a³ + b³ - 2ab(a + b) + ab(a + b) > 0 a³ + b³ - 2ab(a + b) + ab(a + b) > 0 a³ + b³ - 2ab(a + b) + ab(a + b) > 0 a³ + b³ - 2ab(a + b) + ab(a + b) > 0 a³ + b³ - 2ab(a + b) + ab(a + b) > 0 a³ + b³ - 2ab(a + b) + ab(a + b) > 0 a³ + b³ - 2ab(a + b) + ab(a + b) > 0 a³ + b³ - 2ab(a + b) + ab(a + b) > 0 a³ + b³ - 2ab(a + b) + ab(a + b) > 0 a³ + b³ - 2ab(a + b) + ab(a + b) > 0 a³ + b³ - 2ab(a + b) + ab(a + b) > 0 a³ + b³ - 2ab(a + b) + ab(a + b) > 0 a³ + b³ - 2ab(a + b) + ab(a + b) > 0 a³ + b³ - 2ab(a + b) + ab(a + b) > 0 a³ + b³ - 2ab(a + b) + ab(a + b) > 0 a³ + b³ - 2ab(a + b) + ab(a + b) > 0 a³ + b³ - 2ab(a + b) + ab(a + b) > 0 a³ + b³ - 2ab(a + b) + ab(a + b) > 0 a³ + b³ - 2ab(a + b) + ab(a + b) > 0 a³ + b³ - 2ab(a + b) + ab(a + b) > 0 a³ + b³ - 2ab(a + b) + ab(a + b) > 0 a³ + b³ - 2ab(a + b) + ab(a + b) > 0 a³ + b³ - 2ab(a + b) + ab(a + b) > 0 a³ + b³ - 2ab(a + b) + ab(a + b) > 0 a³ + b³ - 2ab(a + b) + ab(a + b) > 0 a³ + b³ - 2ab(a + b) + ab(a + b) > 0 a³ + b³ - 2ab(a + b) + ab(a + b) > 0 a³ + b³ - 2ab(a + b) + ab(a + b) > 0 a³ + b³ - 2ab(a + b) + ab(a + b) > 0 a³ + b³ - 2ab(a + b) + ab(a + b) > 0 a³ + b³ - 2ab(a + b) + ab(a + b) > 0 a³ + b³ - 2ab(a + b) + ab(a + b) > 0 a³ + b³ - 2ab(a + b) + ab(a + b) > 0 a³ + b³ - 2ab(a + b) + ab(a + b) > 0 a³ + b³ - 2ab(a + b) + ab(a + b) > 0 a³ + b³ - 2ab(a + b) + ab(a + b) > 0 a³ + b³ - 2ab(a + b) + ab(a + b) > 0 a³ + b³ - 2ab(a + b) + ab(a + b) > 0 a³ + b³ - 2ab(a + b) + ab(a + b) > 0 a³ + b³ - 2ab(a + b) + ab(a + b) > 0 a³ + b³ - 2ab(a + b) + ab(a + b) > 0 a³ + b³ - 2ab(a + b) + ab(a + b) > 0 a³ + b³ - 2ab(a + b) + ab(a + b) > 0 a³ + b³ - 2ab(a + b) + ab(a + b) > 0 a³ + b³ - 2ab(a + b) + ab(a + b) > 0 a³ + b³ - 2ab(a + b) + ab(a + b) > 0 a³ + b³ - 2ab(a + b) + ab(a + b) > 0 a³ + b³ - 2ab(a + b) + ab(a + b) > 0 a³ + b³ - 2ab(a + b) + ab(a + b) > 0 a³ + b³ - 2ab(a + b) + ab(a + b) > 0 a³ + b³ - 2ab(a + b) + ab(a + b) > 0 a³ + b³ - 2ab(a + b) + ab(a + b) > 0 a³ + b³ - 2ab(a + b) + ab(a + b) > 0 a³ + b³ - 2ab(a + b) + ab(a + b) > 0 a³ + b³ - 2ab(a + b) + ab(a + b) > 0 a³ + b³ - 2ab(a + b) + ab(a + b) > 0 a³ + b³ - 2ab(a + b) + ab(a + b) > 0 a³ + b³ - 2ab(a + b) + ab(a + b) > 0 a³ + b³ - 2ab(a + b) + ab(a + b) > 0 a³ + b³ - 2ab(a + b) + ab(a + b) > 0 a³ + b³ - 2ab(a + b) + ab(a + b) > 0 a³ + b³ - 2ab(a + b) + ab(a + b) > 0 a³ + b³ - 2ab(a + b) + ab(a + b) > 0 a³ + b³ - 2ab(a + b) + ab(a + b) > 0 a³ + b³ - 2ab(a + b) + ab(a + b) > 0 a³ + b³ - 2ab(a + b) + ab(a + b) > 0 a³ + b³ - 2ab(a + b) + ab(a + b) > 0 a³ + b³ - 2ab(a + b) + ab(a + b) > 0 a³ + b³ - 2ab(a + b) + ab(a + b) > 0 a³ + b³ - 2ab(a + b) + ab(a + b) > 0 a³ + b³ - 2ab(a + b) + ab(a + b) > 0 a³ + b³ - 2ab(a + b) + ab(a + b) > 0 a³ + b³ - 2ab(a + b) + ab(a + b) > 0 a³ + b³ - 2ab(a + b) + ab(a + b) > 0 a³ + b³ - 2ab(a + b) + ab(a + b) > 0 a³ + b³ - 2ab(a + b) + ab(a + b) > 0 a³ + b³ - 2ab(a + b) + ab(a + b) > 0 a³ + b³ - 2ab(a + b) + ab(a + b) > 0 a³ + b³ - 2ab(a + b) + ab(a + b) > 0 a³ + b³ - 2ab(a + b) + ab(a + b) > 0 a³ + b³ - 2ab(a + b) + ab(a + b) > 0 a³ + b³ - 2ab(a + b) + ab(a + b) > 0 a³ + b³ - 2ab(a + b) + ab(a + b) > 0 a³ + b³ - 2ab(a + b) + ab(a + b) > 0 a³ + b³ - 2ab(a + b) + ab(a + b) > 0 a³ + b³ - 2ab(a + b) + ab(a + b) > 0 a³ + b³ - 2ab(a + b) + ab(a + b) > 0 a³ + b³ - 2ab(a + b) + ab(a + b) > 0 a³ + b³ - 2ab(a + b) + ab(a + b) > 0 a³ + b³ - 2ab(a + b) + ab(a + b) > 0 a³ + b³ - 2ab(a + b) + ab(a + b) > 0 a³ + b³ - 2ab(a + b) + ab(a + b) > 0 a³ + b³ - 2ab(a + b) + ab(a + b) > 0 a³ + b³ - 2ab(a + b) + ab(a + b) > 0 a³ + b³ - 2ab(a + b) + ab(a + b) > 0 a³ + b³ - 2ab(a + b) + ab(a + b) > 0 a³ + b³ - 2ab(a + b) + ab(a + b) > 0 a³ + b³ - 2ab(a + b) + ab(a + b) > 0 a³ + b³ - 2ab(a + b) + ab(a + b) > 0 a³ + b³ - 2ab(a + b) + ab(a + b) > 0 a³ + b³ - 2ab(a + b) + ab(a + b) > 0 a³ + b³ - 2ab(a + b) + ab(a + b) > 0 a³ + b³ - 2ab(a + b) + ab(a + b) > 0 a³ + b³ - 2ab(a + b) + ab(a + b) > 0 a³ + b³ - 2ab(a + b) + ab(a + b) > 0 a³ + b³ - 2ab(a + b) + ab(a + b) > 0 a³ + b³ - 2ab(a + b) + ab(a + b) > 0 a³ + b³ - 2ab(a + b) + ab(a + b) > 0 a³ + b³ - 2ab(a + b) + ab(a + b) > 0 a³ + b³ - 2ab(a + b) + ab(a + b) > 0 a³ + b³ - 2ab(a + b) + ab(a + b) > 0 a³ + b³ - 2ab(a + b) + ab(a + b) > 0 a³ + b³ - 2ab(a + b) + ab(a + b) > 0 a³ + b³ - 2ab(a + b) + ab(a + b) > 0 a³ + b³ - 2ab(a + b) + ab(a + b) > 0 a³ + b³ - 2ab(a + b) + ab(a + b) > 0 a³ + b³ - 2ab(a + b) + ab(a + b) > 0 a³ + b³ - 2ab(a + b) + ab(a + b) > 0 a³ + b³ - 2ab(a + b) + ab(a + b) > 0 a³ + b³ - 2ab(a + b) + ab(a + b) > 0 a³ + b³ - 2ab(a + b) + ab(a + b) > 0 a³ + b³ - 2ab(a + b) + ab(a + b) > 0 a³ + b³ - 2ab(a + b) + ab(a + b) > 0 a³ + b³ - 2ab(a + b) + ab(a + b) > 0 a³