B=(4:9.x-2:15).(4:9.x-2:15).(4:9.x-2:15).(4:9.x-2:15).(4:9.x-2:15).(4:9.x-2:15)+3
=(4:9.x-2:15)⁶+3
=(4x/9 - 2/15)⁶+3
Vì (4x/9 - 2/15)⁶ >= 0
=> (4x/9 - 2/15)⁶+3 >= 3
Để dấu "=" xảy ra thì:
(4x/9 - 2/15)⁶=0
=> 4x/9 - 2/15 = 0
=> 4x/9 = 2/15
=> x = 2.9/15.4
=> x = 3/10
Vậy Bmin = 3 khi x = 3/10.