ĐKXĐ: x<>0 và y<>0

\(\begin{cases} x^2y + 2y + x = 4xy \quad (1) \\ \dfrac{1}{x^2} + \dfrac{1}{xy} + \dfrac{x}{y} = 3 \quad (2) \end{cases}\)

(1) =>\(\frac{x^2y + 2y + x}{xy} = 4\)

=>\(x+\frac{2}{x}+\frac{1}{y}=4\)

=>\(\left(x+\frac{1}{y}\right)+\frac{2}{x}=4\)

(2) =>\(\frac{1}{x^2} + \frac{1}{y} \left(\frac{1}{x} + x\right) = 3\)

=>\(\frac{1}{x^2}+\frac{1}{y}\cdot\frac{x^2 + 1}{x}=3\)

Đặt \(a=x+\frac{1}{y};b=\frac{1}{x}\)

=>\(\frac{1}{y}=a-x=a-\frac{1}{b}\)

(2) sẽ trở thành: \(b^2 + \left(a - \frac{1}{b}\right)\left(b + \frac{1}{b}\right) = 3\)

=>\(b^2+a\left(b+\frac{1}{b}\right)-1-\frac{1}{b^2}=3\)

=>\(\left(b^2-\frac{1}{b^2}\right)+a\left(b+\frac{1}{b}\right)=4\)

=>\(\left(b+\frac{1}{b}\right)\left(b-\frac{1}{b}\right)+a\left(b+\frac{1}{b}\right)=4\)

=>\(\left(b+\frac{1}{b}\right)\left(b-\frac{1}{b}+a\right)=4\) (3)

\(\left(x+\frac{1}{y}\right)+\frac{2}{x}=4\)

=>a+2b=4

=>a=4-2b

=>\(b - \frac{1}{b} + (4 - 2b) = 4 - \left(b + \frac{1}{b}\right)\)

(3)=>\(\left(b + \frac{1}{b}\right) \left[4 - \left(b + \frac{1}{b}\right)\right] = 4\)

=>\(4\left(b+\frac{1}{b}\right)-\left(b+\frac{1}{b}\right)^2-4=0\)

=>\(\left(b+\frac{1}{b}\right)^2-4\cdot\left(b+\frac{1}{b}\right)+4=0\)

=>\(\left(b+\frac{1}{b}-2\right)^2=0\)

=>\(b+\frac{1}{b}-2=0\)

=>\(\frac{b^2+1-2b}{b}=0\)

=>\(\left(b-1\right)^2=0\)

=>b-1=0

=>b=1

=>\(\frac{1}{x}=1\)

=>x=1

\(a=4-2b=4-2\cdot1=4-2=2\)

\(a=x+\frac{1}{y}\)

=>\(1+\frac{1}{y}=2\)

=>\(\frac{1}{y}=2-1=1\)

=>y=1

ĐKXĐ: \(x;y\ne0\)

\(x^2y+2y+x=4xy\Leftrightarrow x+\frac{2}{x}+\frac{1}{y}=4\)

Đặt \(\left\{{}\begin{matrix}\frac{1}{x}=a\\\frac{1}{y}=b\end{matrix}\right.\) \(\Rightarrow\frac{x}{y}=\frac{b}{a}\) ( hệ trở thành:

\(\left\{{}\begin{matrix}\frac{1}{a}+2a+b=4\\a^2+ab+\frac{b}{a}=3\end{matrix}\right.\) \(\Leftrightarrow\left\{{}\begin{matrix}a+\frac{1}{a}+a+b=4\\a^2+1+b\left(a+\frac{1}{a}\right)=4\end{matrix}\right.\)

\(\Leftrightarrow\left\{{}\begin{matrix}a+\frac{1}{a}+a+b=4\\a\left(a+\frac{1}{a}\right)+b\left(a+\frac{1}{a}\right)=4\end{matrix}\right.\) \(\Leftrightarrow\left\{{}\begin{matrix}a+\frac{1}{a}+a+b=4\\\left(a+\frac{1}{a}\right)\left(a+b\right)=4\end{matrix}\right.\)

\(\Rightarrow\) Theo Viet đảo, \(a+\frac{1}{a}\) và \(a+b\) là nghiệm của:

\(t^2-4t+4=0\Rightarrow t=2\Rightarrow\left\{{}\begin{matrix}a+\frac{1}{a}=2\\a+b=2\end{matrix}\right.\) \(\Leftrightarrow\left\{{}\begin{matrix}a=1\\b=1\end{matrix}\right.\) \(\Rightarrow\left\{{}\begin{matrix}x=1\\y=1\end{matrix}\right.\)

Ta có:

\(\left(x-\frac{1}{y}\right)\left(y+\frac{1}{x}\right)=2\)

\(\Leftrightarrow x^2y^2-2xy-1=0\)

Giải ra tìm được xy thế vô pt sau giải tiếp

ĐKXĐ: ...

\(\Leftrightarrow\left\{{}\begin{matrix}\left(\frac{x}{y+1}\right)^2+\left(\frac{y}{x+1}\right)^2=\frac{1}{2}\\\left(\frac{x}{y+1}\right)\left(\frac{y}{x+1}\right)=\frac{1}{4}\end{matrix}\right.\)

Đặt \(\left\{{}\begin{matrix}\frac{x}{y+1}=u\\\frac{y}{x+1}=v\end{matrix}\right.\)

\(\Rightarrow\left\{{}\begin{matrix}u^2+v^2=\frac{1}{2}\\uv=\frac{1}{4}\end{matrix}\right.\) \(\Leftrightarrow\left\{{}\begin{matrix}u^2+v^2=\frac{1}{2}\\2uv=\frac{1}{2}\end{matrix}\right.\)

\(\Rightarrow u^2-2uv+v^2=0\Rightarrow u=v\)

\(\Rightarrow\left[{}\begin{matrix}u=v=\frac{1}{2}\\u=v=-\frac{1}{2}\end{matrix}\right.\)

\(\Rightarrow\left[{}\begin{matrix}\left\{{}\begin{matrix}\frac{x}{y+1}=\frac{1}{2}\\\frac{y}{x+1}=\frac{1}{2}\end{matrix}\right.\\\left\{{}\begin{matrix}\frac{x}{y+1}=-\frac{1}{2}\\\frac{y}{x+1}=-\frac{1}{2}\end{matrix}\right.\end{matrix}\right.\) \(\Leftrightarrow\left[{}\begin{matrix}\left\{{}\begin{matrix}2x=y+1\\2y=x+1\end{matrix}\right.\\\left\{{}\begin{matrix}2x=-y-1\\2y=-x-1\end{matrix}\right.\end{matrix}\right.\)

\(\Leftrightarrow\left[{}\begin{matrix}\left\{{}\begin{matrix}2x-y=1\\-x+2y=1\end{matrix}\right.\\\left\{{}\begin{matrix}2x+y=-1\\x+2y=-1\end{matrix}\right.\end{matrix}\right.\) bạn tự bấm casio ra kết quả

\(\hept{\begin{cases}y^6+y^3+2x^2=\sqrt{xy-x^2y^2}\left(1\right)\\4xy^3+y^2+\frac{1}{2}\ge2x^2+\sqrt{1+\left(2x-y\right)^2}\left(2\right)\end{cases}}\)

\(VP\left(1\right)=\sqrt{\frac{1}{4}-\left(xy-\frac{1}{2}\right)^2}\le\frac{1}{2}\Rightarrow VT\left(1\right)=y^6+y^3+2x^2\le\frac{1}{2}\)

\(\Leftrightarrow2x^2+2y^3+4x^2\le1\left(3\right)\)

Từ (2)(3) => \(8xy^3+2y^3+2\ge2y^6+4x^2+4x^2+2\sqrt{1+\left(2x-y\right)^2}\)

\(\Leftrightarrow8xy^3+2\ge2y^6+8x^2+2\sqrt{2+\left(2x-y\right)^2}\)

\(\Leftrightarrow4xy^3+1\ge y^6+4x^2+\sqrt{1+\left(2x-y\right)^2}\)

\(\Leftrightarrow1-\sqrt{1+\left(2x-y\right)^2}\ge y^6-4xy^3+4x^2=\left(y^3-2x\right)^2\left(4\right)\)

\(VT\left(4\right)\le0;VP\left(4\right)\ge0\). Do đó:

(4) \(\Leftrightarrow\hept{\begin{cases}y=2x\\y^3=2x\end{cases}\Leftrightarrow\hept{\begin{cases}y=2x\\y^3=y\end{cases}}}\)<=> \(\hept{\begin{cases}x=0\\y=0\end{cases}}\)hoặc \(\hept{\begin{cases}x=\frac{1}{2}\\y=1\end{cases}}\)hoặc \(\hept{\begin{cases}x=\frac{-1}{2}\\y=-1\end{cases}}\)

Thử lại chỉ có \(\left(x;y\right)=\left(\frac{-1}{2};-1\right)\)thỏa mãn

Vậy hệ đã cho có nghiệm duy nhất \(\left(x;y\right)=\left(\frac{-1}{2};-1\right)\)

Hpt cho tương đương:

\(\hept{\begin{cases}xy-x-y+1=6\\\frac{1}{\left(x^2-2x+1\right)-1}+\frac{1}{\left(y^2-2y+1\right)-1}=\frac{2}{3}\end{cases}\Leftrightarrow\hept{\begin{cases}\left(x-1\right)\left(y-1\right)=6\\\frac{1}{\left(x-1\right)^2-1}+\frac{1}{\left(y-1\right)^2-1}=\frac{2}{3}\end{cases}}}\)

Đặt \(x-1=a,y-1=b\)(dễ thấy a,b khác 0). Khi đó hệ trở thành:

\(\hept{\begin{cases}ab=6\\\frac{1}{a^2-1}+\frac{1}{b^2-1}=\frac{2}{3}\end{cases}\Leftrightarrow\hept{\begin{cases}b=\frac{6}{a}\\\frac{1}{a^2-1}+\frac{1}{\frac{36}{a^2}-1}=\frac{2}{3}\left(1\right)\end{cases}}}\)

Giải (1) \(\Leftrightarrow\frac{1}{a^2-1}+\frac{a^2}{36-a^2}=\frac{2}{3}\Leftrightarrow\frac{3\left(36-a^2\right)+3a^2\left(a^2-1\right)}{3\left(a^2-1\right)\left(36-a^2\right)}=\frac{2\left(a^2-1\right)\left(36-a^2\right)}{3\left(a^2-1\right)\left(36-a^2\right)}\)

\(\Rightarrow108-3a^2+3a^4-3a^2=74a^2-2a^4-72\)

\(\Leftrightarrow a^4-16a^2+36=0\Leftrightarrow\left(a^2-8\right)^2=28\Leftrightarrow\orbr{\begin{cases}a^2=8+2\sqrt{7}\\a^2=8-2\sqrt{7}\end{cases}}\)

\(\Leftrightarrow\orbr{\begin{cases}a=\sqrt{8+2\sqrt{7}}\\a=\sqrt{8-2\sqrt{7}}\end{cases}}\Leftrightarrow\orbr{\begin{cases}a=1+\sqrt{7}\\a=1-\sqrt{7}\end{cases}}\)

Suy ra: \(\hept{\begin{cases}a=1+\sqrt{7}\\b=\frac{6}{a}\end{cases}}\) hoặc \(\hept{\begin{cases}a=1-\sqrt{7}\\b=\frac{6}{a}\end{cases}}\)

\(\Leftrightarrow\hept{\begin{cases}a=1+\sqrt{7}\\b=\sqrt{7}-1\end{cases}}\) hoặc \(\hept{\begin{cases}a=1-\sqrt{7}\\b=-1-\sqrt{7}\end{cases}}\)

\(\Rightarrow\hept{\begin{cases}x=2+\sqrt{7}\\y=\sqrt{7}\end{cases}}\) hoặc \(\hept{\begin{cases}x=2-\sqrt{7}\\y=-\sqrt{7}\end{cases}}\). Kết luận:...