ĐK \(xy\ne0\)

\(\Leftrightarrow\hept{\begin{cases}\sqrt{x+4}=2\left(\sqrt{y}+1\right)\\\sqrt{y+4}=2\left(\sqrt{x}+1\right)\end{cases}}\)

\(\Leftrightarrow\hept{\begin{cases}x+4=4\left(y+2\sqrt{y}+1\right)\\y+4=4\left(x+2\sqrt{y}+1\right)\end{cases}}\)

\(\Leftrightarrow\hept{\begin{cases}x=4y+8\sqrt{y}\\y=4x+8\sqrt{x}\end{cases}}\left(I\right)\)

Đặt \(\sqrt{x}=u\left(u\ge o\right),\sqrt{y}=v\left(v\ge0\right)\)

Hệ \(\left(I\right)\)trở thành

\(\hept{\begin{cases}u^2=4v^2+8v\\v^2=4u^2+8u\end{cases}}\)

\(\Rightarrow\left(u^2+v^2\right)-4\left(u^2-v^2\right)=8\left(u-v\right)=0\)

\(\Leftrightarrow\left(u-v\right)\left(-3u-3v-8\right)=0\)

\(\Leftrightarrow\hept{\begin{cases}u=v\Rightarrow x=y=0\\u+v=\frac{8}{3}\end{cases}}\)

P/s nếu làm sai thì thôi nha

\(P=\frac{bc}{2ab+ac}+\frac{ca}{2ab+bc}+\frac{4ab}{bc+ca}\)

Xét \(Q=P+3=\frac{bc}{2ab+ac}+1+\frac{ca}{2ab+bc}+1+\frac{4ab}{bc+ca}+1\)

\(Q=\frac{2ab+ac+bc}{2ab+ac}+\frac{2ab+ac+bc}{2ab+bc}+\frac{4ab+bc+ca}{bc+ca}\)

\(=\left(2ab+ac+bc\right)\left(\frac{1}{2ab+ac}+\frac{1}{2ab+bc}\right)+\frac{4ab+bc+ca}{bc+ca}\)

\(\ge\left(2ab+ac+bc\right)\frac{4}{4ab+ac+bc}+\frac{4ab+bc+ca}{bc+ca}=K\)(Áp dụng BĐT \(\frac{1}{a}+\frac{1}{b}\ge\frac{4}{a+b}\)với a, b không âm)

\(K=\frac{2\left(4ab+ac+bc\right)+2\left(ac+bc\right)}{4ab+ac+bc}+\frac{2\left(4ab+bc+ca\right)}{9\left(ac+bc\right)}\)\(+\frac{7\left(4ab+bc+ca\right)}{9\left(ac+bc\right)}\)

\(=2+\left[\frac{2\left(ac+bc\right)}{4ab+ac+bc}+\frac{2\left(4ab+bc+ca\right)}{9\left(ac+bc\right)}\right]+\frac{7}{9}+\frac{7}{9}.\frac{4ab}{ac+bc}\)

\(\ge2+2\sqrt{\frac{2\left(ac+bc\right)}{4ab+ac+bc}.\frac{2\left(4ab+bc+ca\right)}{9\left(ac+bc\right)}}+\frac{7}{9}+\frac{7}{9}.\frac{4ab}{ac+bc}\)(Áp dụng BĐT Cô - si cho 2 số không âm)

\(=\frac{37}{9}+\frac{7}{9}.\frac{4ab}{ac+bc}\)

Mặt khác: \(6=2\left(\frac{a}{b}+\frac{b}{a}\right)+c\left(\frac{a}{b^2}+\frac{b}{a^2}\right)=\frac{2\left(a^2+b^2\right)}{ab}+\frac{c\left(a^3+b^3\right)}{a^2b^2}\)

\(=\frac{2\left(a^2+b^2\right)}{ab}+\frac{c\left(a+b\right)\left(a^2-ab+b^2\right)}{a^2b^2}\)\(\ge\frac{2.2ab}{ab}+\frac{c\left(a+b\right)\left(2ab-ab\right)}{a^2b^2}=4+\frac{ac+bc}{ab}\)(theo BĐT \(a^2+b^2\ge2ab\))

\(\Rightarrow\frac{ac+bc}{ab}\le2\Leftrightarrow\frac{ab}{ac+bc}\ge\frac{1}{2}\)

\(\Rightarrow K\ge\frac{37}{9}+\frac{7}{9}.\frac{4ab}{ac+bc}\ge\frac{37}{9}+\frac{7}{9}.\frac{4}{2}=\frac{17}{3}\)

Ta có \(Q=P+3\ge K\ge\frac{17}{3}\Rightarrow P\ge\frac{17}{3}-3=\frac{8}{3}\)

Đẳng thức xảy ra khi \(\hept{\begin{cases}2ab+ac=2ab+bc\\\frac{2\left(ac+bc\right)}{4ab+ac+bc}=\frac{2\left(4ab+bc+ca\right)}{9\left(ac+bc\right)}\\a=b\end{cases}}\)\(\Leftrightarrow a=b=c\)

Từ \(2\left(\frac{a}{b}+\frac{b}{a}\right)+c\left(\frac{a}{b^2}+\frac{b}{a^2}\right)=6\Rightarrow6=\frac{c\left(a+b\right)\left(a^2-ab+b^2\right)}{a^2b^2}+\frac{2\left(a^2+b^2\right)}{ab}\)

ta có \(a^2+b^2\ge2ab\Rightarrow6=\frac{c\left(a+b\right)\left(a^2-ab+b^2\right)}{a^2b^2}+\frac{2\left(a^2+b^2\right)}{ab}\ge\frac{c\left(a+b\right)}{ab}+4\)

\(\Rightarrow0< \frac{c\left(a+b\right)}{ab}\le2\)

Lại có 

\(\frac{bc}{a\left(2b+c\right)}+\frac{ac}{b\left(2a+c\right)}=\frac{\left(bc\right)^2}{abc\left(2b+c\right)}+\frac{\left(ac\right)^2}{abc\left(2a+c\right)}\ge\frac{\left(bc+ac\right)^2}{2abc\left(a+b+c\right)}\)\(=\frac{\left[c\left(a+b\right)\right]^2}{2abc\left(a+b+c\right)}\)

và \(abc\left(a+b+c\right)=ab\cdot bc+bc\cdot ba+ab\cdot ca\le\frac{\left(ab+bc+ca\right)^2}{3}\)

\(\Rightarrow\frac{bc}{a\left(2b+c\right)}+\frac{ac}{b\left(2a+c\right)}\ge\frac{3}{2}\left(\frac{c\left(a+b\right)}{ab+bc+ca}\right)^2=\frac{3}{2}\left(\frac{\frac{c\left(a+b\right)}{ab}}{1+\frac{c\left(a+b\right)}{ab}}\right)^2\)

Đặt \(t=\frac{c\left(a+b\right)}{ab}\Rightarrow P\ge\frac{3t^2}{2\left(1+t\right)^2}+\frac{4}{t}\left(0< t\le2\right)\)

Có \(\frac{3t^2}{2\left(1+t\right)^2}+\frac{4}{t}=\left(\frac{3t^2}{\left(1+t\right)^2}+\frac{4}{t}-\frac{8}{3}\right)+\frac{8}{3}=\frac{-7t^2-8t^2+32t+24}{6t\left(1+t\right)^2}+\frac{8}{3}\)

\(=\frac{\left(t-2\right)\left(-7t^2-22t-12\right)}{6t\left(1+t\right)^2}\ge0\forall t\in(0;2]\)

=> \(\frac{\left(t-2\right)\left(-7t^2-22t-12\right)}{6t\left(1+t\right)^2}+\frac{8}{3}\ge\frac{8}{3}\forall t\in(0;2]\frac{1}{2}\)

Dấu "=" xảy ra <=> t=2 hay a=b=c

chuyển vế nhân liên hợp để tạo nhân tử chung là x-y

giải hệ phương trình mình chịu nhe bn

là sao ta

Ta đi giải hệ: \(\hept{\begin{cases}\left(x^2+xy+y^2\right)\sqrt{x^2+y^2}=125\left(1\right)\\\left(x^2-xy+y^2\right)\sqrt{x^2+y^2}=65\left(2\right)\end{cases}}\)

Lấy (1) + (2), ta được: \(\left(x^2+y^2\right)\sqrt{x^2+y^2}=95\Leftrightarrow\sqrt{x^2+y^2}=\sqrt[3]{95}\)

thay vào (1)\(\Rightarrow\left[\left(\sqrt[3]{95}\right)^2+xy\right]\sqrt[3]{95}=125\Rightarrow xy=\frac{125}{\sqrt[3]{95}}-\left(\sqrt[3]{95}\right)^2\)

Từ đó ta có hệ: \(\hept{\begin{cases}x^2+y^2=\sqrt[3]{95}\\2xy=\frac{250}{\sqrt[3]{95}}-2\left(\sqrt[3]{95}\right)^2\end{cases}}\)

Bạn xem lại đề bài chứ giải hệ này ra chắc lên bàn thờ luôn đó!