\(\sqrt{2x+yz}=\sqrt{x\left(x+y+z\right)+yz}=\sqrt{\left(x+y\right)\left(x+z\right)}\le\frac{2x+y+z}{2}\)

cmtt => GTLN

Tìm max:

Ta có:

\(\sqrt{2x+yz}=\sqrt{x\left(x+y+z\right)+xz}=\sqrt{\left(x+y\right)\left(x+z\right)}\)

\(\le\frac{2x+y+z}{2}\left(1\right)\)

Tương tự ta có: \(\hept{\begin{cases}\sqrt{2y+zx}\le\frac{2y+z+x}{2}\left(2\right)\\\sqrt{2z+xy}\le\frac{2z+x+y}{2}\left(3\right)\end{cases}}\)

Cộng (1), (2), (3) vế theo vế ta được

\(A\le\frac{2x+y+z}{2}+\frac{2y+z+x}{2}+\frac{2z+x+y}{2}=2\left(x+y+z\right)=4\)

Dấu = xảy ra khi \(x=y=z=\frac{2}{3}\)

Tìm min:

Ta có: \(\hept{\begin{cases}\sqrt{2x+yz}\ge0\\\sqrt{2y+zx}\ge0\\\sqrt{2z+xy}\ge0\end{cases}}\)

\(\Rightarrow A\ge0\)

Dấu = xảy ra khi \(\left(x,y,z\right)=\left(-2,2,2;2,-2,2;2,2,-2\right)\)

\(\sqrt{2x+yz}=\sqrt{x\left(x+y+z\right)+yz}=\sqrt{\left(x+y\right)\left(x+z\right)}\le\dfrac{1}{2}\left(x+y+x+z\right)=\dfrac{1}{2}\left(2x+y+z\right)\)

Tương tự: \(\sqrt{2y+xz}\le\dfrac{1}{2}\left(x+2y+z\right)\) ; \(\sqrt{2z+xy}\le\dfrac{1}{2}\left(x+y+2z\right)\)

Cộng vế:

\(P\le\dfrac{1}{2}\left(4x+4y+4z\right)=4\)

\(P_{max}=4\) khi \(x=y=z=\dfrac{2}{3}\)

P = \(1.\sqrt{2x+yz}+1.\sqrt{2y+xz}+1.\sqrt{2z+xy}\)

\(\le\sqrt{\left(1^2+1^2+1^2\right)\left(2x+yz+2y+xz+2z+xy\right)}\)

\(=\sqrt{3.\left(4+xy+yz+zx\right)}\)

Đã biết x² + y² + z² \(\ge\)xy + yz + zx

=> xy + yz + zx \(\le\dfrac{\left(x+y+z\right)^2}{3}\)

Khi đó \(P\le\sqrt{3\left(4+xy+yz+zx\right)}\le\sqrt{3\left[4+\dfrac{\left(x+y+z\right)^2}{3}\right]}\)

= 4 

Dấu "=" xảy ra <=> x = 2/3 

\(\sqrt{2x+yz}=\sqrt{\left(x+y+z\right)x+yz}=\sqrt{\left(x+y\right)\left(x+z\right)}\le\dfrac{x+2y+z}{2}\\ \Leftrightarrow P=\sum\sqrt{2x+yz}\le\dfrac{x+2y+z+2x+y+z+x+y+2z}{2}=\dfrac{4\left(x+y+z\right)}{2}=2\cdot2=4\)

Dấu \("="\Leftrightarrow x=y=z=\dfrac{2}{3}\)

P = \(1.\sqrt{2x+yz}+1.\sqrt{2y+xz}+1.\sqrt{2z+xy}\)

\(\le\sqrt{\left(1^2+1^2+1^2\right)\left(2x+yz+2y+xz+2z+xy\right)}\)(Bunyacovski)

\(=\sqrt{3\left[4+\left(xy+yz+zx\right)\right]}\)

\(\le\sqrt{3.\left[4+\dfrac{\left(x+y+z\right)^2}{3}\right]}=\sqrt{3.\left(4+\dfrac{4}{3}\right)}\) = 4

Dấu "=" xảy ra <=> x = y = z = 2/3 

\(-x^2-y^2+xy+2x+2y=-\left[x^2-x\left(y+2\right)+\dfrac{1}{4}\left(y+2\right)^2\right]-\left(\dfrac{3}{4}y^2-3y+3\right)+4=-\left(x-\dfrac{1}{2}y-1\right)^2-\left(\dfrac{\sqrt{3}}{2}y-\sqrt{3}\right)^2+4\le4\)

\(max=4\Leftrightarrow\)\(\left\{{}\begin{matrix}x=2\\y=2\end{matrix}\right.\)

Ta có:

D=2x2+3y2+4xy−8x−2y+18C=2x2+3y2+4xy−8x−2y+18

D=2(x2+2xy+y2)+y2−8x−2y+18C=2(x2+2xy+y2)+y2−8x−2y+18

D=2[(x+y)2−4(x+y)+4]+(y2+6y+9)+1C=2[(x+y)2−4(x+y)+4]+(y2+6y+9)+1

D=2(x+y−2)2+(y+3)2+1≥1C=2(x+y−2)2+(y+3)2+1≥1

Dấu "=" xảy ra ⇔x+y=2⇔x+y=2và y=−3y=−3

Hay x = 5 , y = -3

Đc chx bạn