$#nobody$

`sqrt[x^2 - 12x + 36] + sqrt[x^2 - 16x + 64]`

`= sqrt[(x - 6)^2] + sqrt[(x - 8)^2]`

`= abs(x - 6) + abs(x - 8)`

`= abs(x - 6) + abs(8 - x)`

Áp dụng BĐT GTTĐ ta có:

`abs(x - 6) + abs(8 - x) >= abs(x - 6 + 8 - x) = abs(2) = 2`

Dấu "=" xảy ra `<=> {(x - 6 >= 0),(8 - x >= 0):} <=> 6 <= x <= 8`

Vậy ...