bài 4 : ta có : \(x+2y=3\Leftrightarrow x=3-2y\)

\(\Rightarrow E=x^2+2y^2=\left(3-2y\right)^2+2y^2=4y^2-12y+9+2y^2\)

\(=6y^2-12y+6+3=6\left(y-1\right)^2+3\ge3\)

\(\Rightarrow E_{max}=3\) khi \(x=y=1\)

bài 5 : ta có : \(x^2+3y^2+2xy-10x-14y+18=0\)

\(\Leftrightarrow2y^2-4y+2=-\left(x^2+2xy+y^2\right)+10\left(x+y\right)-16\)

\(\Leftrightarrow2\left(y-1\right)^2=-\left(x+y\right)^2+10\left(x+y\right)-16\ge0\)

\(\Leftrightarrow2\le x+y\le8\)

\(\Rightarrow P_{min}=2\) khi \(\left\{{}\begin{matrix}y=1\\x+y=2\end{matrix}\right.\Leftrightarrow x=y=1\)

\(\Rightarrow P_{max}=8\) khi \(\left\{{}\begin{matrix}y=1\\x+y=8\end{matrix}\right.\Leftrightarrow\left\{{}\begin{matrix}x=7\\y=1\end{matrix}\right.\)

vậy ...........................................................................................................................

Làm nốt phần còn lại của bạn Thắng

(x + y - 5)² + 2(y - 1)² - 9 = 0

<=> 2(y - 1)² = 9 - (S - 5)² \(\ge0\)

\(\Leftrightarrow\left(S-5\right)^2\le9\)

\(\Leftrightarrow-3\le S-5\le3\)

\(\Leftrightarrow2\le S\le8\)

Vậy GTNN là 2 đạt được khi x = y = 1

GTLN là 8 đạt được khi (x, y) = (7, 1)

\(x^2+3y^2+2xy-10x-14y+18\)

\(\Rightarrow\left(x^2+2xy-10x+y^2-10y+25\right)+2y^2-4y-7=0\)

\(\Rightarrow\left(x+y-5\right)^2+2y^2-4y+2-9=0\)

\(\Rightarrow\left(x+y-5\right)^2+2\left(y^2-2y+1\right)-9=0\)

\(\Rightarrow\left(x+y-5\right)^2+2\left(y-1\right)^2-9=0\)

....

x=7;y=±1 và x=y=1 và x=1; y=3 và x=y=3 và x=5;y=-1

Đặt  \(A=-x^2-3y^2-2xy+10x+14y-18\)

Ta có : \(-A=x^2+3y^2+2xy-10x-14y+18\)

\(-A=\left(x^2+2xy+y^2\right)+2y^2-10x-14y+18\)

\(-A=\left[\left(x+y\right)^2-2\left(x+y\right)\times5+25\right]+2y^2-4y+7\)

\(-A=\left(x+y-5\right)^2+2\left(y^2-2y+1\right)+5\)

\(-A=\left(x+y-5\right)^2+2\left(y-1\right)^2+5\)

Mà \(\left(x+y-5\right)^2\ge0\forall x;y\in R\)

\(\left(y-1\right)^2\ge0\forall y\in R\Rightarrow2\left(y-1\right)^2\ge0\forall y\in R\)

\(\Rightarrow-A\ge5\)

\(\Leftrightarrow A\le-5\)

Dấu " = " xảy ra khi:

\(\hept{\begin{cases}x+y-5=0\\y-1=0\end{cases}}\Leftrightarrow\hept{\begin{cases}x=4\\y=1\end{cases}}\)

Vậy Max A = - 5 khi ( x ; y ) = ( 4 ; 1 )

a: \(x^2+3y^2-4x+6y+7=0\)

\(\Leftrightarrow x^2-4x+4+3y^2+6y+3=0\)

\(\Leftrightarrow\left(x-2\right)^2+3\left(y+1\right)^2=0\)

\(\Leftrightarrow\left(x,y\right)=\left(-2;1\right)\)

Ta có: \(4\ge2\left(x^2+y^2\right)\ge\left(x+y\right)^2\)

    \(\Rightarrow x+y\le2\)

Ta có: \(P=\sqrt{x\left(14x+10y\right)}+\sqrt{y\left(14y+10x\right)}\)

              \(=\sqrt{\dfrac{24x\left(14x+10y\right)}{24}}+\sqrt{\dfrac{24y\left(14y+10x\right)}{24}}\le\dfrac{\dfrac{24x+14x+10y}{2}}{\sqrt{24}}+\dfrac{\dfrac{24y+14y+10x}{2}}{\sqrt{24}}\)

\(\Leftrightarrow P\le\dfrac{24\left(x+y\right)}{2\sqrt{6}}\le\dfrac{24.2}{2\sqrt{6}}=4\sqrt{6}\)

Dấu "=" xảy ra ⇔ x = y = 1