` (x - 1)(x - 3)(x - 4)(x - 6) + 9 `
` = [(x - 3)(x - 4)][(x - 1)(x - 6)] + 9 `
` = (x^2 - 7x + 12)(x^2 - 7x + 6) + 9 `
Đặt ` x^2 - 7x + 6 = t, ` ta có:
` (t + 6).t + 9 `
` = t^2 + 6t + 9 `
` = t^2 + 2.t.3 + 3^2 `
` = (t + 3)^2 `
` = (x^2 - 7x + 6 + 3)^2 `
` = (x^2 - 7x + 9)^2 `