a: Thay x=9 vào A, ta được:
\(A=\frac{4\left(3+1\right)}{25-9}=\frac{4\cdot4}{16}=1\)
b: \(B=\left(\frac{15-\sqrt{x}}{x-25}+\frac{2}{\sqrt{x}+5}\right):\frac{\sqrt{x}+1}{\sqrt{x}-5}\)
\(=\left(\frac{15-\sqrt{x}}{\left(\sqrt{x}-5\right)\left(\sqrt{x}+5\right)}+\frac{2}{\sqrt{x}+5}\right)\cdot\frac{\sqrt{x}-5}{\sqrt{x}+1}\)
\(=\frac{15-\sqrt{x}+2\left(\sqrt{x}-5\right)}{\left(\sqrt{x}+5\right)\left(\sqrt{x}-5\right)}\cdot\frac{\sqrt{x}-5}{\sqrt{x}+1}=\frac{\sqrt{x}+5}{\left(\sqrt{x}+5\right)\left(\sqrt{x}+1\right)}=\frac{1}{\sqrt{x}+1}\)
c: P=A*B
\(=\frac{1}{\sqrt{x}+1}\cdot\frac{4\left(\sqrt{x}+1\right)}{25-x}=\frac{4}{25-x}\)
Để P là số nguyên lớn nhất thì 25-x=1
=>x=24
