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Áp dụng BĐT \(\frac{1}{a}+\frac{1}{b}\ge\frac{4}{a+b}\)với a,b>0
Ta có: \(\frac{4xy}{z+1}=\frac{4xy}{2z+x+y}\le\frac{xy}{x+z}+\frac{xy}{y+z}\)
Tương tự: \(\frac{4yz}{x+1}\le\frac{yz}{x+y}+\frac{yz}{x+z}\)
\(\frac{4zx}{y+1}\le\frac{zx}{y+x}+\frac{zx}{y+z}\)
\(\Rightarrow4\left(\frac{xy}{z+1}+\frac{yz}{x+1}+\frac{zx}{y+1}\right)\le\frac{xy}{x+z}+\frac{xy}{y+z}+\frac{yz}{x+y}+\frac{yz}{x+z}+\frac{zx}{y+x}+\frac{zx}{y+z}=x+y+z=1\)
\(\Rightarrow\frac{xy}{z+1}+\frac{yz}{x+1}+\frac{zx}{y+1}\le\frac{1}{4}\)
Dấu "=" xảy ra khi: x=y=z>0
Bài 2:
+) Với y=0 <=> x=0
Ta có: 1-xy= 12 (đúng)
+) Với \(y\ne0\)
Ta có: \(x^6+xy^5=2x^3y^2\)
\(\Leftrightarrow x^6-2x^3y^2+y^4=y^4-xy^5\)
\(\Leftrightarrow\left(x^3-y^2\right)^2=y^4\left(1-xy\right)\)
\(\Rightarrow1-xy=\left(\frac{x^3-y^2}{y^2}\right)^2\)
Với mọi số thực ta luôn có:
`(x-y)^2>=0`
`<=>x^2-2xy+y^2>=0`
`<=>x^2+y^2>=2xy`
`<=>(x+y)^2>=4xy`
`<=>(x+y)^2>=16`
`<=>x+y>=4(đpcm)`
\(\dfrac{1}{x+3}+\dfrac{1}{y+3}=\dfrac{x+3+y+3}{\left(x+3\right)\left(y+3\right)}\)
\(=\dfrac{x+y+6}{3x+3y+13}\)(vì \(xy=4\))
=> \(\dfrac{x+y+6}{3x+3y+13}\)≤\(\dfrac{2}{5}\)
<=> \(5\left(x+y+6\right)\)≤\(2\left(3x+3y+13\right)\)
<=>\(6x+6y+26-5x-5y-30\)≥\(0\)
<=> \(x+y-4\)≥\(0\)
Áp dụng BĐT AM-GM \(\dfrac{a+b}{2}\)≥\(\sqrt{ab}\)
Ta có \(\dfrac{x+y}{2}\)≥\(\sqrt{xy}\)
<=>\(x+y\) ≥ 2\(\sqrt{xy}\)
=>2\(\sqrt{xy}-4\)≥\(0\)
<=> \(4-4\)≥0
<=>0≥0 ( Luôn đúng )
Vậy \(\dfrac{1}{x+3}+\dfrac{1}{y+3}\)≤\(\dfrac{2}{5}\)
\(\dfrac{x-y}{z^2+1}=\dfrac{x-y}{z^2+xy+yz+zx}=\dfrac{x-y}{z\left(z+y\right)+x\left(z+y\right)}=\dfrac{x-y}{\left(x+z\right)\left(z+y\right)}\)
Tương tự: \(\dfrac{y-z}{x^2+1}=\dfrac{y-z}{\left(x+y\right)\left(x+z\right)}\);\(\dfrac{z-x}{y^2+1}=\dfrac{z-x}{\left(x+y\right)\left(y+z\right)}\)
Cộng vế với vế \(\Rightarrow VT=\dfrac{x-y}{\left(x+z\right)\left(y+z\right)}+\dfrac{y-z}{\left(x+y\right)\left(x+z\right)}+\dfrac{z-x}{\left(x+y\right)\left(y+z\right)}\)
\(=\dfrac{\left(x-y\right)\left(x+y\right)+\left(y-z\right)\left(y+z\right)+\left(z-x\right)\left(z+x\right)}{\left(x+y\right)\left(y+z\right)\left(z+x\right)}\)
\(=\dfrac{x^2-y^2+y^2-z^2+z^2-x^2}{\left(x+y\right)\left(y+z\right)\left(z+x\right)}=0\)(đpcm)
\(\frac{1}{xy}+\frac{1}{yz}+\frac{1}{zx}=0\Rightarrow\frac{x+y+z}{xyz}=0\Rightarrow x+y+z=0\Rightarrow x^3+y^3+z^3=3xyz\)
\(N=\frac{x^2}{yz}+\frac{y^2}{zx}+\frac{z^2}{xy}=\frac{x^3+y^3+z^3}{xyz}=\frac{3xyz}{xyz}=3\)
Xét \(xy>1\)
Ta chứng minh: \(\frac{1}{1+x^2}+\frac{1}{1+y^2}-\frac{2}{1+xy}\ge0\)
\(\Leftrightarrow\left(x-y\right)^2\left(xy-1\right)\ge0\)(đúng)
Dấu = xảy ra khi \(x=y\) (loại)
Xét \(xy< 1\)
Ta chứng minh: \(\frac{1}{1+x^2}+\frac{1}{1+y^2}-\frac{2}{1+xy}\le0\)
\(\Leftrightarrow\left(x-y\right)^2\left(xy-1\right)\le0\)(đúng)
Dấu = xảy ra khi \(x=y\) (loại)
Từ (1) và (2) \(\Rightarrow xy=1\)
\(\Rightarrow P=\frac{1}{1+x^2}+\frac{1}{1+y^2}+\frac{2}{1+xy}=\frac{4}{1+xy}=\frac{4}{2}=2\)