\(xy+\sqrt{\left(1+y^2\right)\left(1+x^2\right)}=1\)
c/m\(x\sqrt{1+y^2}+y\sqrt{1+x^2}=0\)
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AH
Akai Haruma
Giáo viên
20 tháng 3 2019
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Câu hỏi của Vũ Sơn Tùng - Toán lớp 9 | Học trực tuyến
21 tháng 7 2018
2
\(A=\sqrt{1-6x+9x^2}+\sqrt{9x^2-12x+4}\)
A= \(\sqrt{9x^2-6x+1}+\sqrt{9x^2-12x+4}\)
A= \(\sqrt{\left(3x-1\right)^2}+\sqrt{\left(3x-2\right)^2}=\left|3x-1\right|+\left|3x-2\right|\)
ta có |3x-1|+|3x-2|=|3x-1|+|2-3x| ≥ |3x-1+2-3x|=1
=> A ≥ 1
=> Min A =1 khi 1/3 ≤ x ≤ 2/3
8 tháng 2 2021
1. ĐKXĐ : \(xy>0\)
Ta có : \(P=\left(\dfrac{\left(\sqrt{x}-\sqrt{y}\right)\left(\sqrt{x}+\sqrt{y}\right)}{\sqrt{x}-\sqrt{y}}+\dfrac{\left(\sqrt{x}-\sqrt{y}\right)\left(x+\sqrt{xy}+y\right)}{-\left(\sqrt{x}-\sqrt{y}\right)\left(\sqrt{x}+\sqrt{y}\right)}\right)\left(\dfrac{\sqrt{x}+\sqrt{y}}{x-2\sqrt{xy}+y+\sqrt{xy}}\right)\)
\(=\dfrac{\left(\sqrt{x}-\sqrt{y}\right)\left(\sqrt{x}+\sqrt{y}\right)\left(\sqrt{x}+\sqrt{y}\right)-\left(\sqrt{x}-\sqrt{y}\right)\left(x+\sqrt{xy}+y\right)}{\left(\sqrt{x}+\sqrt{y}\right)\left(\sqrt{x}-\sqrt{y}\right)}\left(\dfrac{\sqrt{x}+\sqrt{y}}{x-2\sqrt{xy}+y+\sqrt{xy}}\right)\)
\(=\dfrac{\left(\left(\sqrt{x}-\sqrt{y}\right)\left(\sqrt{x}+\sqrt{y}\right)\left(\sqrt{x}+\sqrt{y}\right)-\left(\sqrt{x}-\sqrt{y}\right)\left(x+\sqrt{xy}+y\right)\right)\left(\sqrt{x}+\sqrt{y}\right)}{\left(\sqrt{x}+\sqrt{y}\right)\left(\sqrt{x}-\sqrt{y}\right)\left(x-\sqrt{xy}+y\right)}\)
\(=\dfrac{\left(\sqrt{x}-\sqrt{y}\right)\left(\sqrt{x}+\sqrt{y}\right)\left(\sqrt{x}+\sqrt{y}\right)-\left(\sqrt{x}-\sqrt{y}\right)\left(x+\sqrt{xy}+y\right)}{\left(\sqrt{x}-\sqrt{y}\right)\left(x-\sqrt{xy}+y\right)}\)
\(=\dfrac{\left(\sqrt{x}+\sqrt{y}\right)^2-\left(x+\sqrt{xy}+y\right)}{x-\sqrt{xy}+y}=\dfrac{x+2\sqrt{xy}+y-x-\sqrt{xy}-y}{x-\sqrt{xy}+y}\)
\(=\dfrac{\sqrt{xy}}{x-\sqrt{xy}+y}\)
2. Ta thấy : \(x-\sqrt{xy}+y=x-\dfrac{2.\sqrt{x}.\sqrt{y}}{2}+\dfrac{y}{4}+\dfrac{3y}{4}\)
\(=\left(\sqrt{x}-\dfrac{\sqrt{y}}{2}\right)^2+\dfrac{3y}{4}\)
Mà \(\left\{{}\begin{matrix}\left(\sqrt{x}-\dfrac{\sqrt{y}}{2}\right)^2\ge0\\\dfrac{3y}{4}\ge0\end{matrix}\right.\)
\(\Rightarrow x-\sqrt{xy}+y\ge0\)
Lại có : \(\sqrt{xy}\ge0\)
\(\Rightarrow P\ge0\) ( ĐPCM )
20 tháng 3 2019
b)\(\sqrt{5x^2+2xy+2y^2}+\sqrt{2x^2+2xy+5y^2}=3\left(x+y\right)\)
\(\Rightarrow\left(\sqrt{5x^2+2xy+2y^2}+\sqrt{2x^2+2xy+5y^2}\right)^2=\left(3\left(x+y\right)\right)^2\)
\(\Leftrightarrow\sqrt{\left(5x^2+2xy+2y^2\right)\left(2x^2+2xy+5y^2\right)}=x^2+7xy+y^2\)
\(\Rightarrow\left(5x^2+2xy+2y^2\right)\left(2x^2+2xy+5y^2\right)=\left(x^2+7xy+y^2\right)^2\)
\(\Leftrightarrow9\left(x-y\right)^2\left(x+y\right)^2=0\)\(\Leftrightarrow\left[{}\begin{matrix}x=y\\x=-y\end{matrix}\right.\)
\(\rightarrow\left(x;y\right)\in\left\{\left(0;0\right),\left(1;1\right)\right\}\)
\(xy+\sqrt{\left(1+y^2\right)\left(1+x^2\right)}=1\)
\(\Leftrightarrow\sqrt{\left(1+y^2\right)\left(1+x^2\right)}=1-xy\)
\(\Leftrightarrow\left(1+y^2\right)\left(1+x^2\right)=1+x^2y^2-2xy\)
\(\Leftrightarrow1+x^2+y^2+x^2y^2=1+x^2y^2-2xy\)
\(\Leftrightarrow x^2+y^2=-2xy\)
\(\Leftrightarrow x^2+y^2+2xy=0\)
\(\Leftrightarrow\left(x+y\right)^2=0\)
\(\Leftrightarrow x=-y\)
Thay vào ,ta có
\(x\sqrt{1+y^2}+y\sqrt{1+x^2}=-y\sqrt{1+x^2}+y\sqrt{1+x^2}=0\)(đpcm)
đây là cách của mk
@-@
Ta có \(1=\left(xy+\sqrt{\left(1+y^2\right)\left(1+x^2\right)}\right)^2\)
\(=x^2y^2+\left(1+y^2\right)\left(1+x^2\right)+2xy\sqrt{\left(1+y^2\right)\left(1+x^2\right)}\)
\(=x^2y^2+1+x^2+y^2+x^2y^2+2xy\sqrt{\left(1+y^2\right)\left(1+x^2\right)}\)
\(=x^2\left(1+y^2\right)+y^2\left(1+x^2\right)+2xy\sqrt{\left(1+y^2\right)\left(1+x^2\right)}+1\)
\(\Leftrightarrow x^2\left(1+y^2\right)+y^2\left(1+x^2\right)+2xy\sqrt{\left(1+y^2\right)\left(1+x^2\right)}=0\)
\(\Leftrightarrow\left(x\sqrt{1+y^2}+y\sqrt{1+x^2}\right)^2=0\)
\(\Rightarrow x\sqrt{1+y^2}+y\sqrt{1+x^2}=0\)