Ta có:

\(\dfrac{12x-15y}{2017}=\dfrac{20z-12x}{2018}=\dfrac{15y-20z}{2019}\)

\(=\dfrac{12x-15y+20z-12x+15y-20z}{2017+2018+2019}\)

\(=\dfrac{0}{2017+2018+2019}=0\)

\(\Rightarrow\left\{{}\begin{matrix}12x-15y=0\\20z-12x=0\\15y-20z=0\end{matrix}\right.\)\(\Rightarrow\left\{{}\begin{matrix}12x=15y\\20z=12x\\15y=20z\end{matrix}\right.\)\(\Rightarrow12x=15y=20z\)

\(\Rightarrow\dfrac{x}{5}=\dfrac{y}{4}=\dfrac{z}{3}\)

Áp dụng tích chất dãy tỉ số bằng nhau, ta có:

\(\dfrac{x}{5}=\dfrac{y}{4}=\dfrac{z}{3}=\dfrac{x+y+z}{5+4+3}=\dfrac{48}{12}=4\)

\(\Rightarrow\left\{{}\begin{matrix}x=4.5=20\\y=4.4=16\\z=4.3=12\end{matrix}\right.\)

Vậy ...

Áp dụng TCDTSBN ta có:

\(\dfrac{x+y+2017}{z}=\dfrac{y+z-2018}{x}=\dfrac{z+x+1}{y}=\dfrac{x+y+2017+y+z-2018+z+x+1}{z+x+y}=\dfrac{2x+2y+2z}{x+y+z}=\dfrac{2\left(x+y+z\right)}{x+y+z}=2\)

\(\dfrac{z+x+1}{y}=\dfrac{2}{x+y+z};\dfrac{z+x+1}{y}=2\\ \Rightarrow\dfrac{2}{x+y+z}=2\\ \Rightarrow x+y+z=1\)

\(\left\{{}\begin{matrix}\dfrac{x+y+2017}{z}=2\\\dfrac{y+z-2018}{x}=2\\\dfrac{z+x+1}{y}=2\end{matrix}\right.\\ \Leftrightarrow\left\{{}\begin{matrix}x+y+2017=2z\\y+z-2018=2x\\z+x+1=2y\end{matrix}\right.\\ \Leftrightarrow\left\{{}\begin{matrix}x+y+z=3z-2017\\y+z+x=3x+2018\\z+x+y=3y-1\end{matrix}\right.\\ \Leftrightarrow\left\{{}\begin{matrix}3z-2017=1\\3x+2018=1\\3y-1=1\end{matrix}\right.\\ \Leftrightarrow\left\{{}\begin{matrix}3z=2018\\3x=-2017\\3y=2\end{matrix}\right.\)

\(\Leftrightarrow\left\{{}\begin{matrix}z=\dfrac{2018}{3}\\x=\dfrac{-2017}{3}\\y=\dfrac{2}{3}\end{matrix}\right.\)

Vậy \(\left\{{}\begin{matrix}x=\dfrac{-2017}{3}\\y=\dfrac{2}{3}\\z=\dfrac{2018}{3}\end{matrix}\right.\)

Hình như là sai đề bn ak!

Áp dụng BĐT Cauchy–Schwarz ta được:

\(x=\dfrac{2017}{\sqrt{2018}}+\dfrac{2018}{\sqrt{2017}}\ge\dfrac{\left(\sqrt{2018}+\sqrt{2017}\right)^2}{\sqrt{2018}+\sqrt{2017}}=\sqrt{2018}+\sqrt{2017}=y\)

Dấu \("="\Leftrightarrow\dfrac{2017}{\sqrt{2018}}=\dfrac{2018}{\sqrt{2017}}\Leftrightarrow2017=2018\left(vô.lí\right)\)

Vậy đẳng thức ko xảy ra hay \(x>y\)

Áp dụng tính chất của dãy tỉ số bằng nhau, ta được:

\(\dfrac{x-1}{2018}=\dfrac{3-y}{2019}=\dfrac{x-1+3-y}{2018+2019}=1\)

=>x-1=2018 và 3-y=2019

=>x=2019; y=-2016

\(\dfrac{x}{2017}=\dfrac{y}{2018}=\dfrac{z}{2019}=k\\ \Rightarrow\left\{{}\begin{matrix}x=2017k\\y=2018k\\z=2019k\end{matrix}\right.\)

\(4\left(x-y\right)\left(y-z\right)=4\left(2017k-2018k\right)\left(2018k-2019k\right)=4\left(-k\right)\left(-k\right)=4k^2=\left(2k\right)^2=\left(2019k-2017k\right)^2=\left(z-x\right)^2\left(ĐPCM\right)\)

a. ĐKXĐ: \(x\ge-1\)

\(y=\sqrt{x^3+1+2\sqrt{x^3+1}+1}+\sqrt{x^3+1-2\sqrt{x^3+1}+1}\)

\(=\sqrt{\left(\sqrt{x^3+1}+1\right)^2}+\sqrt{\left(\sqrt{x^3+1}-1\right)^2}\)

\(=\left|\sqrt{x^3+1}+1\right|+\left|1-\sqrt{x^3+1}\right|\ge\left|\sqrt{x^3+1}+1+1-\sqrt{x^3+1}\right|=2\)

b.

\(f\left(x\right)=\dfrac{x-1}{2}+\dfrac{2}{x-1}+\dfrac{1}{2}\ge2\sqrt{\dfrac{2\left(x-1\right)}{2\left(x-1\right)}}+\dfrac{1}{2}=\dfrac{5}{2}\)

c.

\(y=\dfrac{x-2018+1}{\sqrt{x-2018}}=\sqrt{x-2018}+\dfrac{1}{\sqrt{x-2018}}\ge2\sqrt{\dfrac{\sqrt{x-2018}}{\sqrt{x-2018}}}=2\)

Chứng minh Nesbit 4 số rồi áp dụng nhé 

\(\frac{a}{b+c}+\frac{b}{c+d}+\frac{c}{d+a}+\frac{d}{a+b}=\frac{a^2}{a\left(b+c\right)}+\frac{b^2}{b\left(c+d\right)}+\frac{c^2}{c\left(d+a\right)}+\frac{d^2}{d\left(a+b\right)}\)  (*)

Theo Cauchy - Schwarz dạng engel , ta có 

(*) \(\ge\frac{\left(a+b+c+d\right)^2}{a\left(b+c\right)+b\left(c+d\right)+c\left(d+a\right)+d\left(a+b\right)}\) 

\(=\frac{2\left(a+c\right)\left(b+d\right)+\left(a+c\right)^2+\left(b+d\right)^2}{\left(a+c\right)\left(b+d\right)+2ac+2bd}\ge\frac{2\left(a+c\right)\left(b+d\right)+4ac+4bd}{\left(a+c\right)\left(b+d\right)+2ac+2bd}=2\)

Đẳng thức xảy ra <=> a = c và b = d 

Áp dụng bất đẳng thức Nesbit cho 4 số ,ta có 

\(\frac{2018}{x+y}+\frac{x}{y+2017}+\frac{y}{2017+2018}+\frac{2017}{x+2018}\ge2\)

Đẳng thức xảy ra <=> y = 2018 , x = 2017