1: Thay x=25 vào A, ta được:\(A=\dfrac{5-2}{5+2}=\dfrac{3}{7}\)2: Ta có: \(B=\dfrac{\sqrt{x}+2}{\sqrt{x}-2}-\dfrac{3}{\sqrt{x}+2}-\dfrac{12}{x-4}\)\(=\dfrac{x+4\sqrt{x}+4-3\sqrt{x}+6-12}{\left(\sqrt{x}-2\right)\left(\sqrt{x}+2\right)}\)\(=\dfrac{x+\sqrt{x}-2}{\left(\sqrt{x}-2\right)\left(\sqrt{x}+2\right)}\)\(=\dfrac{\sqrt{x}-1}{\sqrt{x}-2}\)Câu trả lời: 1: Thay x=25 vào A, ta được:\(A=\dfrac{5-2}{5+2}=\dfrac{3}{7}\)2: Ta có: \(B=\dfrac{\sqrt{x}+2}{\sqrt{x}-2}-\dfrac{3}{\sqrt{x}+2}-\dfrac{12}{x-4}\)\(=\dfrac{x+4\sqrt{x}+4-3\sqrt{x}+6-12}{\left(\sqrt{x}-2\right)\left(\sqrt{x}+2\right)}\)\(=\dfrac{x+\sqrt{x}-2}{\left(\sqrt{x}-2\right)\left(\sqrt{x}+2\right)}\)\(=\dfrac{\sqrt{x}-1}{\sqrt{x}-2}\)