Có: \(\frac{x^2+y^2}{xy}=\frac{25}{12}\)

\(\Rightarrow x^2+y^2=\frac{25xy}{12}\)

Có: \(P=\frac{x-y}{x+y}\)

\(\Rightarrow P^2=\frac{x^2+y^2-2xy}{x^2+y^2+2xy}=\frac{\frac{25xy}{12}-2xy}{\frac{25xy}{12}+2xy}=\frac{\frac{xy}{12}}{\frac{49xy}{12}}=\frac{1}{49}\)

VÌ: \(x< y< 0\Rightarrow x-y< 0;x+y< 0\)

=> \(P>0\)

=> \(P=\frac{1}{7}\)

\(\Leftrightarrow\hept{\begin{cases}3\left(x^2+y^2\right)=10xy\left(1\right)\\x< y< 0\end{cases}}\)  \(\Rightarrow\hept{\begin{cases}xy>0\\x-y>0\\x+y< 0\end{cases}}\)  \(\Rightarrow P< 0\)(*)

\(\left(1\right)\Rightarrow\hept{\begin{cases}3\left(x-y\right)^2=4xy\left(2\right)\\3\left(x+y\right)^2=16xy\left(3\right)\end{cases}}\)

\(\frac{\left(1\right)}{\left(2\right)}=\frac{\left(x-y\right)^2}{\left(x+y\right)^2}=\frac{1}{4}\Rightarrow\orbr{\begin{cases}\frac{x-y}{x+y}=\frac{1}{2}\\\frac{x-y}{x+y}=-\frac{1}{2}\end{cases}}\)

Từ (*)=> P=-1/2

\(\frac{x^2+y^2}{xy}=\frac{25}{12}\)

\(\Rightarrow12x^2+12y^2=25xy\)

\(\Rightarrow12x^2+12y^2+24xy=49xy\)

\(\Rightarrow12\left(x^2+2xy+y^2\right)=49xy\)

\(\Rightarrow\left(x+y\right)^2=\frac{49xy}{12}\)

\(\Rightarrow x+y=\sqrt{\frac{49xy}{12}}\)

Lại có :\(12\left(x^2-2xy+y^2\right)=xy\)

\(\Rightarrow x-y=\sqrt{\frac{xy}{12}}\)

\(\Rightarrow A=\sqrt{\frac{\frac{xy}{12}}{\frac{49xy}{12}}}\)

\(\Rightarrow A=\sqrt{\frac{1}{49}}=\pm\frac{1}{7}\)

Phạm Tuấn Đạt Chỉ kiến thức lớp 7 là đủ rồi bạn ey!À mà \(\sqrt{\frac{1}{49}}=-\frac{1}{7}???\) không có căn bậc 2 của số âm nha bạn!

\(\frac{x^2+y^2}{xy}=\frac{25}{12}\Leftrightarrow\frac{x^2+y^2}{25}=\frac{xy}{12}\)

Đặt \(\frac{x^2+y^2}{25}=\frac{xy}{12}=k\Rightarrow x^2+y^2=25k;xy=12k\)

\(A^2=\frac{\left(x-y\right)^2}{\left(x+y\right)^2}=\frac{x^2-2xy+y^2}{x^2+2xy+y^2}=\frac{25k-2.12k}{25k+2.12k}=\frac{25k-24k}{25k+24k}=\frac{1k}{49k}=\frac{1}{49}\)

\(\Rightarrow A=\sqrt{\frac{1}{49}}=\frac{1}{7}\)

à mình nhầm sorry bạn! còn bài mình thiếu : \(\sqrt{\frac{1}{49}}=\pm\frac{1}{7}\)!=))

\(\frac{x^2+y^2}{xy}=\frac{25}{12}\Rightarrow12\left(x^2+y^2\right)=25xy\)

\(\Rightarrow12x^2+12y^2-25xy=0\Rightarrow12x\left(x-2y\right)-y\left(x-2y\right)=0\Rightarrow\left(12x-y\right)\left(x-2y\right)=0\)

\(x< y< 0\Rightarrow12x< y\Rightarrow12x-y< 0\)

Do đó: \(x-2y=0\Rightarrow x=2y\)

Vậy \(A=\frac{x-y}{x+y}=\frac{2y-y}{2y+y}=\frac{1}{3}\)

Ta có : \(\frac{x^2+y^2}{xy}=\frac{25}{12}\)

\(\Leftrightarrow\frac{x^2+2xy+y^2-2xy}{xy}=\frac{25}{12}\)

\(\Leftrightarrow\frac{\left(x+y\right)^2-2xy}{xy}=\frac{25}{12}\)

\(\Rightarrow xy=12\)(cùng mẫu )

\(\Leftrightarrow\left(x+y\right)^2-2.12=25\)

\(\Leftrightarrow\left(x+y\right)^2=49\)

\(\Leftrightarrow x+y=7\)

Mà \(\hept{\begin{cases}x+y=7\\x.y=12\\x< y\end{cases}}\Rightarrow\hept{\begin{cases}x=3\\y=4\end{cases}}\)

\(\Rightarrow A=\frac{x-y}{x+y}=\frac{3-4}{3+4}=-\frac{1}{7}\)

\(x< y< 0\) mà bạn leminhduc ơi; 3>0; 4>0

4x + 2x = 68 – 2^19 : 2^16 5x + x = 39 – 3^11:3^9

Ta có : \(x+y+z=0\Rightarrow x+y=-z\)

\(\Rightarrow\left(x+y\right)^2=z^2\Rightarrow x^2+y^2+2xy=z^2\)

\(\Rightarrow x^2+y^2=z^2-2xy\)

Tương tự ta có : \(y^2+z^2=x^2-2yz\)

\(x^2+z^2=y^2-2xz\)

Thay vào biểu thức ta có :

\(A=\frac{x^2}{y^2+z^2-x^2}+\frac{y^2}{x^2+z^2-y^2}+\frac{z^2}{x^2+y^2-z^2}\)

\(=\frac{x^2}{x^2-2yz-x^2}+\frac{y^2}{y^2-2xz-y}+\frac{z^2}{z^2-2xy-z^2}\)

\(=-\frac{x^2}{2yz}-\frac{y^2}{2xz}-\frac{z^2}{2xy}\)

\(=\frac{-x^3-y^3-z^3}{2xyz}=-\frac{x^3+y^3+z^3}{2xyz}\)

\(=\frac{3xyz}{2xyz}=-\frac{3}{2}\)

Chỗ \(x^3+y^3+z^3=3xyz\)là do \(x+y+z=0\)nhé, bạn cần chứng minh không ?

\(x+y+z=0\Rightarrow x+y=-z\)

\(\Rightarrow\left(x+y\right)^2=\left(-z\right)^2\Rightarrow x^2+2xy+y^2=z^2\Rightarrow x^2+y^2-z^2=-2xy\)

Tương tự: \(y^2+z^2-x^2=-2yz,x^2+z^2-y^2=-2xz\)

\(\frac{1}{y^2+z^2-x^2}+\frac{1}{x^2+y^2-z^2}+\frac{1}{x^2+z^2-y^2}\)

\(=\frac{1}{-2yz}+\frac{1}{-2xy}+\frac{1}{-2xz}=\frac{x+y+z}{-2xyz}=0\)

\(\frac{x^2+y^2}{xy}=\frac{10}{3}\Rightarrow3x^2+3y^2-10xy=0\)

\(\Rightarrow\left(3x^2-9xy\right)-\left(xy-3y^2\right)=0\Rightarrow3x\left(x-3y\right)-y\left(x-3y\right)=0\)

\(\Rightarrow\left(x-3y\right)\left(3x-y\right)=0\Rightarrow3x-y=0\left(y>x>0\Rightarrow x-3y< 0\right)\Rightarrow3x=y\)

\(M=\frac{x-y}{x+y}=\frac{x-3x}{x+3x}=\frac{-2x}{4x}=-\frac{1}{2}\)