PTDTTNT bang cach dat an (6x+5)^2(3x+2)(x+1)-35
Đặt 6x + 5 =t
ta có: 3x + 2 = (6x + 4)/2 = (6x + 5 - 1)/2 = (t - 1)/2
x + 1 = (6x + 6)/6 = (6x + 5 + 1)/6 = (t + 1) /6
Lại Có:
ĐẶT B = t²(t - 1)(t + 1)/12 - 35 = t²(t² - 1)/12 - 35
12B = (t²)² - t² - 420 = (t²)² - 400 - t² - 20 = (t² - 20)(t² + 20) - (t² + 20)
12B = (t² + 20)(t² - 21) = ((6x + 5)² + 20)((6x + 5)² - 21)
12B = (36x² + 60x + 45)(36x² + 60x + 4)
12B = 3.4.(12x² + 20x + 15)(9x² + 15x + 1)
=> B = (12x² + 20x + 15)(9x² + 15x + 1)