[laTEX] 4 \leq x \leq 6 \\ \\ \sqrt{x-4} -1 + \sqrt{6-x} -1 = 2x^2-13x+15 \\ \\ \frac{x-5}{\sqrt{x-4} +1} - \frac{x-5}{\sqrt{6-x} +1} = (2x-3)(x-5) \\ \\ TH_1: x = 5 \\ \\ TH_2: \frac{1}{\sqrt{x-4} +1} - \frac{1}{\sqrt{6-x} +1} = 2x-3 \\ \\ f(x) = VT \\ \\ f'(x) < 0 \forall x \in [4,6] \Rightarrow Max VT = f(4) = 1 - \frac{1}{\sqrt{2}+1} \\ \\ Min VP = 2.4-3 = 5 > MaxVT \Rightarrow TH_2: vo-nghiem [/laTEX]