Áp dụng tính chất : 1/a+b < = 1/4.(1/a+1/b) thì :
x/2x+y+z = x.(1/2x+y+z) = x.[1/(x+y)+(x+z)] < = x/4.(1/x+y + 1/x+z)
Tương tự : ..........
=> x/2x+y+z + y/x+2y+z + z/x+y+2z < = 1/4.(x/x+y + x/x+z + y/y+x + y/y+z + z/z+x + z/x+y )
= 1/4. [ ( x/x+y + y/x+y ) + ( y/y+z + z/z+y ) + ( z/z+x + x/x+z )
= 1/4.(1+1+1) = 3/4
Dấu "=" xảy ra <=> x=y=z
Vậy ..........
Tk mk nha
Đặt BT là P:
\(\text{P}=\frac{x}{\left(2x+y+z\right)}-1+\frac{y}{2y+z+x}-1+\frac{z}{\left(2z+x+y\right)}-1+3\)
\(\text{P}=-\frac{\left(x+y+z\right)}{\left(2x+y-z\right)}-\frac{\left(x+y+z\right)}{\left(2y+z+x\right)}-\frac{\left(x+y+z\right)}{\left(2z+x+y\right)}+3\)
\(\text{P}=-\left(x+y+z\right).\left[\frac{1}{\left(2x+y+z\right)}+\frac{1}{\left(2y+z+x\right)}+\frac{1}{\left(2z+x+y\right)}\right]+3\)
Co-si 3 số, ta có:
\(2x+y+z+2y+z+x+2z+x+y\ge3.\sqrt[3]{\left(2x+y+z\right)\left(2y+z+x\right)\left(2z+x+y\right)}\)
\(\Rightarrow4\left(x+y+z\right)\ge3.\sqrt[3]{\left(2x+y+z\right)\left(2y+z+x\right)\left(2z+x+y\right)}\)(1)
Co-si tiếp cho 3 số, ta có:
\(\frac{1}{\left(2x+y+z\right)}+\frac{1}{\left(2y+z+x\right)}+\frac{1}{\left(2z+x+y\right)}\ge3.\sqrt[3]{\frac{1}{\left(2x+y+z\right)}+\frac{1}{\left(2y+z+x\right)}+\frac{1}{\left(2z+x+y\right)}}\)(2)
Lấy (1) và (2) ta có: \(4\left(x+y+z\right)\left[\frac{1}{\left(2x+y+z\right)}+\frac{1}{\left(2y+z+x\right)}+\frac{1}{\left(2z+x+y\right)}\right]\ge9\)
\(\Rightarrow-\left(x+y+z\right).\left[\frac{1}{\left(2x+y+z\right)}+\frac{1}{\left(2y+z+x\right)}+\frac{1}{\left(2z+x+y\right)}\right]\le-\frac{9}{4}\)
Thay P, ta có:
\(\text{P}\le-\frac{9}{3}+3=\frac{3}{4}\left(ĐPCM\right)\)
Dấu "=" xảy ra khi x = y = z.
