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Đặt \(t=x^2+\left(3-x\right)^2\Rightarrow t\ge5\)
Mặt khác: \(t=x^2+\left(3-x\right)^2=9-2x\left(3-x\right)\Rightarrow x\left(3-x\right)=\frac{9-t}{2}\)
Ta có: \(P=\left[x^2+\left(3-x\right)^2\right]^2+4x^2\left(3-x\right)^2=t^2+4\left(\frac{9-t}{2}\right)^2\)
\(=2t^2-18t+81=2\left(t-\frac{9}{2}\right)^2+\frac{81}{2}\)
Mà \(t\ge5\Rightarrow t-\frac{9}{2}\ge\frac{1}{2}\Rightarrow P\ge2.\left(\frac{1}{2}\right)^2+\frac{81}{2}=41\)
Đẳng thức xảy ra khi \(t=5\Leftrightarrow x^2+\left(3-x\right)^2=5\Leftrightarrow x^2-3x+2\Leftrightarrow\orbr{\begin{cases}x=1\\x=2\end{cases}}\)
Vậy \(MinP=41\), đạt được khi \(x\in\left\{1;2\right\}\)
Đặt \(x^2+\left(3-x\right)^2=a\ge5\)
Ta có:
\(x\left(3-x\right)=-\frac{1}{2}\left(2x^2-6x\right)\)
\(=-\frac{1}{2}\left(x^2-6x+9+x^2-9\right)\)
\(=-\frac{1}{2}\left(x^2+\left(3-x\right)^2-9\right)=-\frac{1}{2}\left(a-9\right)\)
Áp dụng ta có:
\(P=x^4+\left(3-x\right)^4+6x^2\left(3-x\right)^2=\left(x^2+\left(3-x\right)^2\right)^2+4x^2\left(3-x\right)^2\)
\(=a^2+\left(a-9\right)^2\)
\(=2a^2-18a+81=\left(2a^2-20a+50\right)+2a+31\)
\(=2\left(a-5\right)^2+2a+31\ge0+2.5+31=41\)
Đặt \(\left\{{}\begin{matrix}\sqrt{2x+3}=a\ge0\\\sqrt{y}=b\ge0\end{matrix}\right.\)
\(\Rightarrow b\left(b^2+1\right)-3a^2=\left(a^2+1\right)a-3b^2\)
\(\Rightarrow a^3-b^3+3a^2-3b^2+a-b=0\)
\(\Leftrightarrow\left(a-b\right)\left(a^2+ab+b^2\right)+\left(a-b\right)\left(3a+3b\right)+a-b=0\)
\(\Leftrightarrow\left(a-b\right)\left(a^2+ab+b^2+3a+3b+1\right)=0\)
\(\Leftrightarrow a=b\Rightarrow\sqrt{2x+3}=\sqrt{y}\)
\(\Rightarrow y=2x+3\)
\(\Rightarrow M=x\left(2x+3\right)+3\left(2x+3\right)-4x^2-3\) tới đây chắc chỉ cần bấm máy
- \(B=\left(4x^2+3y\right)\left(4y^2+3x\right)+25xy=16x^2y^2+12\left(x^3+y^3\right)+34xy\)
\(=16x^2y^2+12\left(x+y\right)\left(x^2-xy+y^2\right)+34xy\)
\(=16x^2y^2+12\left[\left(x+y\right)^2-2xy\right]+22xy\)
\(=16x^2y^2-2xy+12\)
Đặt \(t=xy\) thì \(B=16t^2-2t+12=16\left(t-\frac{1}{16}\right)^2+\frac{191}{16}\ge\frac{191}{16}\)
Đẳng thức xảy ra khi \(\hept{\begin{cases}x+y=1\\xy=\frac{1}{16}\end{cases}\Leftrightarrow}\hept{\begin{cases}x=\frac{2+\sqrt{3}}{4}\\y=\frac{2-\sqrt{3}}{4}\end{cases}}\) hoặc \(\hept{\begin{cases}x=\frac{2-\sqrt{3}}{4}\\y=\frac{2+\sqrt{3}}{4}\end{cases}}\)
Vậy min B \(=\frac{191}{16}\) khi \(\left(x;y\right)=\left(\frac{2+\sqrt{3}}{4};\frac{2-\sqrt{3}}{4}\right);\left(\frac{2-\sqrt{3}}{4};\frac{2+\sqrt{3}}{4}\right)\)
- Như trên ta có : \(B=16\left(xy-\frac{1}{16}\right)^2+\frac{191}{16}\)
Mặt khác, áp dụng BĐT Cauchy , ta có : \(1=x+y\ge2\sqrt{xy}\Rightarrow xy\le\frac{1}{4}\)
Suy ra : \(B\le16\left(\frac{1}{4}-\frac{1}{16}\right)^2+\frac{191}{16}=\frac{25}{2}\)
Đẳng thức xảy ra khi x = y = 1/2
Vậy max B = 25/2 khi (x;y) = (1/2;1/2)
\(\Leftrightarrow6\left(\dfrac{x}{y}+\dfrac{y}{x}\right)+20=\dfrac{5\left(x+y\right)\left(xy+3\right)}{xy}\ge\dfrac{5\left(x+y\right)2\sqrt{3xy}}{xy}=10\sqrt{3}\left(\sqrt{\dfrac{x}{y}}+\sqrt{\dfrac{y}{x}}\right)\)
Đặt \(\sqrt{\dfrac{x}{y}}+\sqrt{\dfrac{y}{x}}=t\ge2\Rightarrow\dfrac{x}{y}+\dfrac{y}{x}=t^2-2\)
\(\Rightarrow6\left(t^2-2\right)+20\ge10\sqrt{3}t\)
\(\Rightarrow3t^2-5\sqrt{3}t+4\ge0\)
\(\Rightarrow\left(\sqrt{3}t-1\right)\left(\sqrt{3}t-4\right)\ge0\)
Do \(t\ge2\Rightarrow\sqrt{3}t-1>0\)
\(\Rightarrow\sqrt{3}t-4\ge0\Rightarrow t\ge\dfrac{4}{\sqrt{3}}\)
\(\Rightarrow t^2\ge\dfrac{16}{3}\Rightarrow t^2-2\ge\dfrac{10}{3}\)
\(\Rightarrow\dfrac{x}{y}+\dfrac{y}{x}\ge\dfrac{10}{3}\) (do \(\dfrac{x}{y}+\dfrac{y}{x}=t^2-2\))
Vậy \(A_{min}=\dfrac{10}{3}\) khi \(\left(x;y\right)=\left(1;3\right);\left(3;1\right)\)
Đặt \(y=3-x\).Ta có:\(\hept{\begin{cases}x+y=3\\x^2+y^2\ge5\end{cases}\Leftrightarrow\hept{\begin{cases}x^2+y^2+2xy=9\\x^2+y^2\ge5\end{cases}}}\)
\(\Rightarrow x^2+y^2+4\left(x^2+y^2+2xy\right)\ge5+4.9=41\)
\(\Rightarrow5\left(x^2+y^2\right)+4\left(2xy\right)\ge41\)
Mặt khác \(16\left(x^2+y^2\right)^2+25\left(2xy\right)^2\ge40\left(x^2+y^2\right)\left(2xy\right)\left(1\right)\)
Cộng 2 vế của (1) với \(25\left(x^2+y^2\right)^2+16\left(2xy\right)^2\):
\(\Rightarrow41\left[\left(x^2+y^2\right)^2+\left(2xy\right)^2\right]\ge\left[5\left(x^2+y^2\right)+4\left(2xy\right)^2\right]\ge41\)
hay \(\left(x^2+y^2\right)^2+\left(2xy\right)^2\ge41\Leftrightarrow x^4+y^4+6x^2y^2\ge41\)
Vậy minP=41
You ơi , you thiếu điều kiện xảy ra dấu "="