Cho x,y,z dương sao : \(\frac{1}{xy}+\frac{1}{yz}+\frac{1}{zx}=2019\)
CMR: \(\frac{x^2+1+\sqrt{2019x^2+1}}{x}+\frac{y^2+1+\sqrt{2019y^2+1}}{y}+\frac{z^2+1+\sqrt{2019z^{ 2}+1}}{z}\le2019.2020xyz\)
K
Khách
Hãy nhập câu hỏi của bạn vào đây, nếu là tài khoản VIP, bạn sẽ được ưu tiên trả lời.
Những câu hỏi liên quan
KN
18 tháng 2 2020
Ta có: \(\frac{1}{xy}+\frac{1}{yz}+\frac{1}{zx}=2019\)
\(\Rightarrow\frac{x+y+z}{xyz}=2019\)
\(\Rightarrow x+y+z=2019xyz\)
\(\Rightarrow2019x^2=\frac{x^2+xy+xz}{yz}\)
\(\Rightarrow2019x^2+1=\frac{x^2+xy+xz+yz}{yz}=\frac{\left(x+y\right)\left(x+z\right)}{yz}\)
\(=\left(\frac{x}{y}+1\right)\left(\frac{x}{z}+1\right)\)
\(\Rightarrow\sqrt{2019x^2+1}=\sqrt{\left(\frac{x}{y}+1\right)\left(\frac{x}{z}+1\right)}\)\(\le\frac{1}{2}\left(\frac{x}{y}+\frac{x}{z}+2\right)=1+\frac{x}{2}\left(\frac{1}{y}+\frac{1}{z}\right)\)(cô -si)
\(\Rightarrow\frac{x^2+1+\sqrt{2019x^2+1}}{x}\le\frac{x^2+1+1+\frac{x}{2}\left(\frac{1}{y}+\frac{1}{z}\right)}{x}\)\(=x+\frac{2}{x}+\frac{1}{2}\left(\frac{1}{y}+\frac{1}{z}\right)\)
Tương tự ta có: \(\frac{y^2+1+\sqrt{2019y^2+1}}{y}\le y+\frac{2}{y}+\frac{1}{2}\left(\frac{1}{z}+\frac{1}{x}\right)\)
và \(\frac{z^2+1+\sqrt{2019z^2+1}}{z}\le z+\frac{2}{z}+\frac{1}{2}\left(\frac{1}{x}+\frac{1}{y}\right)\)
Cộng từng vế của các bđt trên, ta được:
\(\text{Σ}_{cyc}\frac{x^2+1+\sqrt{2019x^2+1}}{x}\le x+y+z+3\left(\frac{1}{x}+\frac{1}{y}+\frac{1}{z}\right)\)
Chứng minh được: \(\left(x+y+z\right)^2\ge3\left(xy+yz+zx\right)\)
\(\Rightarrow3\left(\frac{1}{x}+\frac{1}{y}+\frac{1}{z}\right)=\frac{3\left(xy+yz+zx\right)}{xyz}=\frac{2019.3\left(xy+yz+zx\right)}{2019xyz}\)
\(\le\frac{2019\left(x+y+z\right)^2}{x+y+z}=2019\left(x+y+z\right)\)
\(\Rightarrow VT\le2020\left(x+y+z\right)=2020.2019xyz\)
Vậy \(\text{Σ}_{cyc}\frac{x^2+1+\sqrt{2019x^2+1}}{x}\le2019.2020xyz\left(đpcm\right)\)
21 tháng 3 2020
Theo bài ra ta có:
\(\frac{1}{xy}+\frac{1}{yz}+\frac{1}{zx}=\frac{z}{xyz}+\frac{x}{xyz}+\frac{y}{xyz}=\frac{x+y+z}{xyz}=2019\)
\(\Rightarrow x+y+z=2019xyz\)
\(\Rightarrow2019x^2=\frac{x^2+xy+xz}{yz}\)
\(\Rightarrow2019x^2+1=\frac{x^2+xy+xz+yz}{yz}=\frac{\left(x+y\right)\left(x+z\right)}{yz}=\left(\frac{x}{y}+1\right)\left(\frac{x}{z}+1\right)\)
\(\Rightarrow\sqrt{2019x^2+1}=\sqrt{\left(\frac{x}{y}+1\right)\left(\frac{x}{z}+1\right)}\le\frac{1}{2}\left(\frac{x}{y}+\frac{x}{z}+2\right)=1+\frac{x}{2}\left(\frac{1}{y}+\frac{1}{z}\right)\)(Theo BĐT Cosi)
\(\Rightarrow\frac{x^2+1+\sqrt{2019^2+1}}{x}\le\frac{x+1+1+\frac{x}{2}\left(\frac{1}{y}+\frac{1}{z}\right)}{x}=x+\frac{2}{x}+\frac{1}{2}\left(\frac{1}{y}+\frac{1}{z}\right)\)
Tương tự:
\(\frac{y^2+1+\sqrt{2019y^2+1}}{y}\le y+\frac{2}{y}+\frac{1}{2}\left(\frac{1}{z}+\frac{1}{x}\right)\)
\(\frac{z^2+1+\sqrt{2019z^2+1}}{z}\le z+\frac{2}{z}+\frac{1}{2}\left(\frac{1}{x}+\frac{1}{y}\right)\)
\(\Rightarrow VT\le x+y+z+3\left(\frac{1}{x}+\frac{1}{y}+\frac{1}{z}\right)\)
Chứng minh được: \(\left(x+y+z\right)^2\ge3\left(xy+yz+zx\right)\)
\(\Rightarrow3\left(\frac{1}{x}+\frac{1}{y}+\frac{1}{z}\right)=\frac{3\left(xy+yz+zx\right)}{xyz}=\frac{2019\cdot3\left(xy+yz+zx\right)}{2019xyz}\le\frac{2019\left(x+y+z\right)^2}{x+y+z}\)\(=2019\left(x+y+z\right)\)
\(\Rightarrow VT\le2020\left(x+y+z\right)=2020\cdot2019xyz=VP\)
=> ĐPCM
I
21 tháng 3 2020
ta có \(x+y+z=2019xyz=>2019x^2=\frac{x^2+xy+xz}{yz}\)
\(=>2019x^2+1=\frac{x^2+xy+xz+yz}{yz}=\frac{\left(x+y\right)\left(x+z\right)}{yz}=\left(\frac{x}{y}+1\right)\left(\frac{x}{z}+1\right)\)
\(=>\sqrt{2019x^2+1}=\sqrt{\left(\frac{x}{y}+1\right)\left(\frac{x}{z}+1\right)}\le\frac{1}{2}\left(\frac{x}{y}+\frac{x}{z}+2\right)=1+\frac{x}{2}\left(\frac{1}{y}+\frac{1}{z}\right)\)
(theo BDT cô -si)
\(=>\frac{x^2+1+\sqrt{2019x^2+1}}{x}\le\frac{x^2+1+1+\frac{x}{2}\left(\frac{1}{y}+\frac{1}{z}\right)}{x}=x+\frac{2}{x}+\frac{1}{2}\left(\frac{1}{y}+\frac{1}{z}\right)\)
tương tự \(\frac{y^2+1+\sqrt{2019y^2+1}}{z}\le y+\frac{2}{y}+\frac{1}{2}\left(\frac{1}{z}+\frac{1}{x}\right)\)
\(\frac{z^2+1+\sqrt{2019z^2+1}}{z}\le z+\frac{2}{z}+\frac{1}{2}\left(\frac{1}{x}+\frac{1}{y}\right)\)
=>.vt\(\le x+y+z+3\left(\frac{1}{x}+\frac{1}{y}+\frac{1}{z}\right)\)
chứng minh được \(\left(x+y+z\right)^2\ge3\left(xy+yz+zx\right)\)
=>\(3\left(\frac{1}{x}+\frac{1}{y}+\frac{1}{z}\right)=\frac{3\left(xy+yz+zx\right)}{2019xyz}\le\frac{2019\left(x+y+z\right)^2}{x+y+z}=2019\left(x+y+z\right)\)
=>.vt\(\le2020\left(x+y+z\right)=2020.2019xyz=\)vt
=> dpcm
21 tháng 3 2020
Ta có: \(2019xyz=x+y+z\)
=> \(2019xy=\frac{x}{z}+\frac{y}{z}+1>1\); \(2019yz=\frac{y}{x}+\frac{z}{x}+1>1\); \(2019xz=\frac{x}{y}+\frac{z}{y}+1>1\)
Ta lại có: \(x+y+z=2019xyz\)
=> \(2019x\left(x+y+z\right)=2019^2x^2yz\)
=> \(2019x^2+1=\left(2019^2x^2yz-2019xy\right)-\left(2019xz-1\right)\)
=> \(2019x^2+1=\left(2019xy-1\right)\left(2019xz-1\right)\le\frac{\left(2019xy+2019xz-2\right)^2}{4}\)
=> \(\sqrt{2019x^2+1}\le\frac{2019xy+2019xz-2}{2}\)
Tương tự : \(\sqrt{2019y^2+1}\le\frac{2019xy+2019yz-2}{2}\)
\(\sqrt{2019z^2+1}\le\frac{2019xz+2019yz-2}{2}\)
=> \(\frac{x^2+1+\sqrt{2019x^2+1}}{x}+\frac{y^2+1+\sqrt{2019y^2+1}}{y}+\frac{z^2+1+\sqrt{2019z^2+1}}{z}\)
\(\le\)\(\frac{x^2+1+\frac{2019xy+2019xz-2}{2}}{x}+\frac{y^2+1+\frac{2019xy+2019yz-2}{2}}{y}+\frac{z^2+1+\frac{2019xz+2019yz-2}{2}}{z}\)
\(=\frac{2x^2+2019xy+2019xz}{2x}+\frac{2y^2+2019xy+2019yz}{2y}+\frac{2z^2+2019xz+2019yz}{2z}\)
\(=x+\frac{2019}{2}y+\frac{2019}{2}z+y+\frac{2019}{2}x+\frac{2019}{2}z+z+\frac{2019}{2}x+\frac{2019}{2}y\)
\(=2020\left(x+y+z\right)=2020.2019xyz\)
Vậy có điều cần cm
Dấu "=" xảy ra <=> \(\hept{\begin{cases}x=y=z\\x+y+z=2019xyz\end{cases}}\Leftrightarrow x=y=z=\frac{1}{\sqrt{673}}\)
T
24 tháng 10 2019
Nhìn qua thấy bậc của bđt là không đồng bậc nên hơi căng đấy...
Chú ý: \(2019=\frac{1}{xy}+\frac{1}{yz}+\frac{1}{zx}=\frac{x+y+z}{xyz}\Rightarrow xyz=\frac{x+y+z}{2019}\)
\(LHS=\Sigma_{cyc}\frac{\sqrt{2019x^2+1}+1}{x}=\Sigma_{cyc}\frac{\sqrt{\frac{x}{y}+\frac{x^2}{yz}+\frac{x}{z}+1}+1}{x}\)( thay \(2019=\frac{1}{xy}+\frac{1}{yz}+\frac{1}{zx}\))
\(=\Sigma_{cyc}\frac{\sqrt{\left(\frac{x}{y}+1\right)\left(\frac{x}{z}+1\right)}+1}{x}=\Sigma_{cyc}\left[\sqrt{\frac{\left(\frac{x}{y}+1\right)}{x}.\frac{\left(\frac{x}{z}+1\right)}{x}}+\frac{1}{x}\right]\)
\(=\Sigma_{cyc}\sqrt{\left(\frac{1}{y}+\frac{1}{x}\right)\left(\frac{1}{z}+\frac{1}{x}\right)}+\frac{1}{x}+\frac{1}{y}+\frac{1}{z}\le\frac{1}{2}\left[4\left(\frac{1}{x}+\frac{1}{y}+\frac{1}{z}\right)\right]+\frac{1}{x}+\frac{1}{y}+\frac{1}{z}\)
\(=3\left(\frac{1}{x}+\frac{1}{y}+\frac{1}{z}\right)=\frac{3\left(xy+yz+zx\right)}{xyz}=\frac{3\left(xy+yz+zx\right)}{\frac{\left(x+y+z\right)}{2019}}=\frac{6057\left(xy+yz+zx\right)}{x+y+z}\)
\(\le\frac{6057.\frac{\left(x+y+z\right)^2}{3}}{x+y+z}=2019\left(x+y+z\right)\)(đpcm)
Đẳng thức xảy ra khi \(x=y=z=\sqrt{\frac{3}{2019}}\)
P/s: Check hộ t phát:3
24 tháng 10 2019
Đặt \(a=\frac{1}{x};b=\frac{1}{y};c=\frac{1}{z}\)thì bài toán thành
Cho: \(ab+bc+ca=2019\)
Chứng minh:
\(\sqrt{2019+a^2}+\sqrt{2019+b^2}+\sqrt{2019+c^2}+\left(a+b+c\right)\le2019\left(\frac{1}{a}+\frac{1}{b}+\frac{1}{c}\right)\)
Ta có:
\(VT=\sqrt{ab+bc+ca+a^2}+\sqrt{ab+bc+ca+b^2}+\sqrt{ab+bc+ca+c^2}+\left(a+b+c\right)\)
\(VT=\sqrt{\left(a+b\right)\left(a+c\right)}+\sqrt{\left(b+a\right)\left(b+c\right)}+\sqrt{\left(c+a\right)\left(c+b\right)}+\left(a+b+c\right)\)
\(\le3\left(a+b+c\right)\)
\(VP=\left(ab+bc+ca\right)\left(\frac{1}{a}+\frac{1}{b}+\frac{1}{c}\right)\)
\(=2\left(a+b+c\right)+\left(\frac{ab}{c}+\frac{bc}{a}+\frac{ca}{b}\right)\)
\(\ge3\left(a+b+c\right)\)
Tới đây bí :(
NV
Nguyễn Việt Lâm
Giáo viên
7 tháng 4 2019
\(VT=\sum\frac{x}{\sqrt{1+x^2}}=\sum\frac{x}{\sqrt{xy+xz+yz+x^2}}=\sum\frac{x}{\sqrt{\left(x+y\right)\left(x+z\right)}}\le\frac{1}{2}\sum\left(\frac{x}{x+y}+\frac{x}{x+z}\right)\)\(\Rightarrow VT\le\frac{1}{2}\left(\frac{x}{x+y}+\frac{x}{x+z}+\frac{y}{y+z}+\frac{y}{x+y}+\frac{z}{x+z}+\frac{z}{y+z}\right)=\frac{3}{2}\)
Dấu "=" xảy ra khi \(x=y=z=\frac{1}{\sqrt{3}}\)
NV
Nguyễn Việt Lâm
Giáo viên
26 tháng 2 2020
\(\sqrt{\frac{xy}{xy+z}}=\sqrt{\frac{xy}{xy+z\left(x+y+z\right)}}=\sqrt{\frac{xy}{\left(x+z\right)\left(y+z\right)}}\le\frac{1}{2}\left(\frac{x}{x+z}+\frac{y}{y+z}\right)\)
Tương tự: \(\sqrt{\frac{yz}{yz+x}}\le\frac{1}{2}\left(\frac{y}{x+y}+\frac{z}{x+z}\right)\) ; \(\sqrt{\frac{zx}{zx+y}}\le\frac{1}{2}\left(\frac{x}{x+y}+\frac{z}{y+z}\right)\)
Cộng vế với vế ta có đpcm
Dấu "=" xảy ra khi \(x=y=z=\frac{1}{3}\)
TN
17 tháng 1 2017
Bài 1:Áp dụng C-S dạng engel
\(\frac{3}{xy+yz+xz}+\frac{2}{x^2+y^2+z^2}=\frac{6}{2\left(xy+yz+xz\right)}+\frac{2}{x^2+y^2+z^2}\)
\(\ge\frac{\left(\sqrt{6}+\sqrt{2}\right)^2}{\left(x+y+z\right)^2}=\left(\sqrt{6}+\sqrt{2}\right)^2>14\)
Theo giả thiết, ta có: \(\frac{1}{xy}+\frac{1}{yz}+\frac{1}{zx}=2019\Rightarrow x+y+z=2019xyz\)
Xét \(\sqrt{2019x^2+1}=\sqrt{\frac{x^2+xy+xz}{yz}+1}=\sqrt{\frac{\left(x+y\right)\left(x+z\right)}{yz}}\le\frac{\frac{x+y}{y}+\frac{x+z}{z}}{2}=\frac{x}{2}\left(\frac{1}{y}+\frac{1}{z}\right)+1\)\(\Rightarrow\frac{x^2+1+\sqrt{2019x^2+1}}{x}\le\frac{x^2+1+\frac{x}{2}\left(\frac{1}{y}+\frac{1}{z}\right)+1}{x}=x+\frac{1}{2}\left(\frac{1}{y}+\frac{1}{z}\right)+\frac{2}{x}\)
Tương tự, ta có: \(\frac{y^2+1+\sqrt{2019y^2+1}}{y}\le y+\frac{1}{2}\left(\frac{1}{z}+\frac{1}{x}\right)+\frac{2}{y}\); \(\frac{z^2+1+\sqrt{2019z^2+1}}{z}\le z+\frac{1}{2}\left(\frac{1}{x}+\frac{1}{y}\right)+\frac{2}{z}\)
Cộng theo vế ba bất đẳng thức trên, ta được: \(\frac{x^2+1+\sqrt{2019x^2+1}}{x}+\frac{y^2+1+\sqrt{2019y^2+1}}{y}+\frac{z^2+1+\sqrt{2019z^2+1}}{z}\le\left(x+y+z\right)+3\left(\frac{1}{x}+\frac{1}{y}+\frac{1}{z}\right)=2019xyz+3.\frac{xy+yz+zx}{xyz}\le2019xyz+3.\frac{\frac{\left(x+y+z\right)^2}{3}}{xyz}=2019xyz+\frac{\left(x+y+z\right)^2}{xyz}=2019xyz+2019^2xyz=2019.2020xyz\)Đẳng thức xảy ra khi \(x=y=z=\frac{1}{\sqrt{673}}\)