Question

[Minh Bình]

Cho A= \(\dfrac{2}{\sqrt{x}-3}+\dfrac{1}{\sqrt{x}+3}\)và B= \(\dfrac{\sqrt{x}+1}{\sqrt{x}-3}\)

a) biết P=B:A. Rút gọn P

b) Tìm x để P có giá trị là số nguyên

Answer 1

[Toru]

\(a,P=B:A\)

\(=\left(\dfrac{\sqrt{x}+1}{\sqrt{x}-3}\right):\left(\dfrac{2}{\sqrt{x}-3}+\dfrac{1}{\sqrt{x}+3}\right)\left(ĐKXĐ:x\ge0;x\ne9\right)\)

\(=\left(\dfrac{\sqrt{x}+1}{\sqrt{x}-3}\right):\left[\dfrac{2\left(\sqrt{x}+3\right)+\sqrt{x}-3}{\left(\sqrt{x}-3\right)\left(\sqrt{x}+3\right)}\right]\)

\(=\left(\dfrac{\sqrt{x}+1}{\sqrt{x}-3}\right):\left[\dfrac{3\sqrt{x}+3}{\left(\sqrt{x}-3\right)\left(\sqrt{x}+3\right)}\right]\)

\(=\dfrac{\sqrt{x}+1}{\sqrt{x}-3}\cdot\dfrac{\left(\sqrt{x}-3\right)\left(\sqrt{x}+3\right)}{3\left(\sqrt{x}+1\right)}\)

\(=\dfrac{\sqrt{x}+3}{3}\)

\(b,\) Để \(P=\dfrac{\sqrt{x}+3}{3}\) có giá trị nguyên

thì \(\sqrt{x}+3⋮3\)

\(\Leftrightarrow\sqrt{x}+3\in B\left(3\right)\)

\(\Leftrightarrow\sqrt{x}\in B\left(3\right)\) 

Kết hợp với điều kiện, ta được:

\(P\) nguyên khi \(x=m^2\left(m\in Z;m⋮3;m\ne3\right)\)

#Toru

Answer 2

[Nguyễn Lê Phước Thịnh]

a: 

ĐKXĐ: x>=0; x<>9

\(A=\dfrac{2\sqrt{x}+6+\sqrt{x}-3}{\left(x-9\right)}=\dfrac{3\sqrt{x}+3}{x-9}\)

\(P=B:A=\dfrac{\sqrt{x}+1}{\sqrt{x}-3}\cdot\dfrac{x-9}{3\left(\sqrt{x}+1\right)}=\dfrac{\sqrt{x}+3}{3}\)

b: P nguyên khi \(\sqrt{x}+3⋮3\)

=>\(\sqrt{x}\in B\left(3\right)\)

=>\(x=k^2\left(k\in Z;k⋮3\right)\)