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![](https://rs.olm.vn/images/avt/0.png?1311)
Ta có: \(P=\frac{\sqrt{x}}{1+x+xy}+\frac{\sqrt{y}}{1+y+yz}+\frac{\sqrt{z}}{1+z+xz}\)
\(P=\frac{\sqrt{x}}{xy+x+1}+\frac{x\sqrt{y}}{x+xy+xyz}+\frac{xy\sqrt{z}}{xy+xyz+x^2yz}\)
\(P=\frac{\sqrt{x}}{xy+x+1}+\frac{x\sqrt{y}}{xy+x+1}+\frac{\sqrt{xy}.\sqrt{xyz}}{xy+x+1}\)
\(P=\frac{\sqrt{x}+x\sqrt{y}+\sqrt{xy}}{xy+x+1}\le\frac{\frac{x+1}{2}+\frac{x\left(y+1\right)}{2}+\frac{xy+1}{2}}{xy+x+1}\) (bđt cosi)
=> \(P\le\frac{x+1+xy+x+xy+1}{2\left(xy+x+1\right)}=\frac{2\left(xy+x+1\right)}{2\left(xy+x+1\right)}=1\)
Dấu "=" xảy ra<=> x = y = z = 1
Vậy MaxP = 1 <=> x = y = z = 1
![](https://rs.olm.vn/images/avt/0.png?1311)
\(Q=\frac{1}{xy}+\frac{1}{yz}+\frac{1}{xz}=\frac{1^2}{xy}+\frac{1^2}{yz}+\frac{1^2}{xz}\ge\frac{\left(1+1+1\right)^2}{xy+yz+xz}\)
\(=\frac{9}{xy+yz+zx}\ge\frac{9}{x^2+y^2+z^2}\ge\frac{9}{6}=\frac{3}{2}\).
Dấu " = " xảy ra <=> x = y =z = \(\sqrt{2}\).
![](https://rs.olm.vn/images/avt/0.png?1311)
\(P+3=\frac{xy}{1+x+y}+1+\frac{yz}{1+y+z}+1+\frac{xz}{1+x+z}+1\)
\(\frac{xy}{1+x+y}+1=\frac{\left(x+1\right)\left(y+1\right)}{1+x+y}\)
\(P+3=\left(x+1\right)\left(y+1\right)\left(z+1\right)\left(\frac{1}{\left(z+1\right)\left(x+y+1\right)}+\frac{1}{\left(y+1\right)\left(x+z+1\right)}+\frac{1}{\left(x+1\right)\left(y+z+1\right)}\right)\)
\(P+3\ge\left(xyz+xy+xz+yz+1\right)\left(\frac{9}{xy+xz+x+y+z+1+xy+yz+x+y+z+1+xz+yz+x+y+z+1}\right)\)
dòng cuối cùng sai, sửa :
\(P+3\ge\left(xyz+xy+xz+yz+1\right)\left(\frac{9}{xy+xz+x+y+z+1+xy+yz+x+y+z+1+xz+yz+x+y+z+1}\right)\)
\(P+3\ge\left(3xyz+xy+xz+yz\right)\left(\frac{9}{2\left(3xyz+xy+xz+yz\right)}\right)=\frac{9}{2}\)
\(P\ge\frac{3}{2}\)
dấu "=" xảy ra <=> x=y=z=\(\frac{1+\sqrt{3}}{2}\)
![](https://rs.olm.vn/images/avt/0.png?1311)
Ta có 1 + x2 = xy + yz + xz + x2 = (xy + x2) + (yz + xz) = (x + y)(x + z)
=> \(1x\sqrt{\frac{\left(1+y^2\right)\left(1+z^2\right)}{\left(1+x^2\right)}}=\:x\sqrt{\frac{\left(y+x\right)\left(y+z\right)\left(z+x\right)\left(z+y\right)}{\left(x+y\right)\left(x+z\right)}}=\:x\left|y+z\right|\)
Tương tự như vậy thì ta có
A = xy + xz + yx + yz + zx + zy = 2
![](https://rs.olm.vn/images/avt/0.png?1311)
Bài này thì có 2 cách Làm cách cồng kềnh nhất vậy :))
\(M=x^3\left(\frac{1}{xy+9}+\frac{1}{xz+9}\right)+y^3\left(\frac{1}{xy+9}+\frac{1}{yz+9}\right)+z^3\left(\frac{1}{yz+9}+\frac{1}{xz+9}\right)\)
C-S ; ta được : \(\frac{1}{xy+9}+\frac{1}{xz+9}\ge\frac{4}{x\left(y+z\right)+18}=\frac{4}{x\left(9-x\right)+18}=\frac{4}{3x+27-\left(x-3\right)^2}\ge\frac{4}{3x+27}\)
Suy ra : \(M\ge\frac{4}{3}\) . sigma \(\frac{x^3}{x+9}\)
Tiếp tục AD C-S ; ta được : \(\frac{x^3}{x+9}+\frac{3}{16}\left(x+9\right)+\frac{9}{4}\ge\frac{9}{4}x\Rightarrow\frac{x^3}{x+9}\ge\frac{33}{16}x-\frac{63}{16}\)
=> sig ma \(\frac{x^3}{x+9}\ge\frac{33}{16}\left(x+y+z\right)-\frac{63}{16}.3=\frac{27}{4}\)
Suy ra : M \(\ge\frac{4}{3}.\frac{27}{4}=9\)
" = " <=> x = y = z = 3
Xong film
Ta có : \(\left(x^2+y^2+z^2\right)\left(1^2+1^2+1^2\right)\le\left(x.1+y.1+z.1\right)^2\) (bđt Bunhiacopxki)
\(\Leftrightarrow x^2+y^2+z^2\le\frac{\left(x+y+z\right)^2}{3}\) hay \(1\le\frac{\left(x+y+z\right)^2}{3}\)
\(\Rightarrow\left(x+y+z\right)^2\ge3\Rightarrow x+y+z\ge\sqrt{3}\) (do x;y;z dương)
Áp dụng bđt AM - GM ta có :
\(\frac{xy}{z}+\frac{yz}{x}\ge2\sqrt{\frac{xy}{z}.\frac{yz}{x}}=2y\)
\(\frac{xy}{z}+\frac{xz}{y}\ge2\sqrt{\frac{xy}{z}.\frac{xz}{y}}=2x\)
\(\frac{yz}{x}+\frac{xz}{y}\ge2\sqrt{\frac{yz}{x}.\frac{xz}{y}}=2z\)
Cộng vế với vế ta được :
\(2C\ge2\left(x+y+z\right)=2\sqrt{3}\Rightarrow C\ge\sqrt{3}\)
Dấu "=" xảy ra \(\Leftrightarrow x=y=z=\frac{1}{\sqrt{3}}\)
Đức Hùng hình như áp dụng sai ( ngược dấu ) BĐT Bunhiacopxki rồi