1: Thay x=16 vào A, ta được:
\(A=\dfrac{4+2}{4-5}=\dfrac{6}{-1}=-6\)
2: \(B=\dfrac{3}{\sqrt{x}+5}+\dfrac{20-2\sqrt{x}}{x-25}\)
\(=\dfrac{3}{\sqrt{x}+5}+\dfrac{20-2\sqrt{x}}{\left(\sqrt{x}+5\right)\left(\sqrt{x}-5\right)}\)
\(=\dfrac{3\left(\sqrt{x}-5\right)+20-2\sqrt{x}}{\left(\sqrt{x}+5\right)\left(\sqrt{x}-5\right)}\)
\(=\dfrac{\sqrt{x}+5}{\left(\sqrt{x}+5\right)\left(\sqrt{x}-5\right)}=\dfrac{1}{\sqrt{x}-5}\)
3: \(A>B\left(2\sqrt{x}+5\right)\)
=>\(\dfrac{\sqrt{x}+2}{\sqrt{x}-5}>\dfrac{2\sqrt{x}+5}{\sqrt{x}-5}\)
=>\(\dfrac{\sqrt{x}+2-2\sqrt{x}-5}{\sqrt{x}-5}>0\)
=>\(\dfrac{-\sqrt{x}-3}{\sqrt{x}-5}>0\)
=>\(\dfrac{\sqrt{x}+3}{\sqrt{x}-5}< 0\)
=>\(\sqrt{x}-5< 0\)
=>\(\sqrt{x}< 5\)
=>0<=x<25
