Biết \(\dfrac{1}{x}+\dfrac{1}{y}=2\). Hãy tìm giá trị lớn nhất của \(D=\dfrac{1}{x+2y}+\dfrac{1}{2x+y}\)
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CL
0
16 tháng 3 2022
Theo bđt Cauchy schwarz dạng Engel
\(P\ge\frac{\left(2x+2y+\frac{1}{x}+\frac{1}{y}\right)^2}{1+1}=\frac{\left[2\left(x+y\right)+\frac{1}{x}+\frac{1}{y}\right]^2}{2}\)
Ta có \(\frac{1}{x}+\frac{1}{y}\ge\frac{4}{x+y}\)(bđt phụ)
\(\Rightarrow P\ge\frac{\left[2.1+4\right]^2}{2}=\frac{36}{2}=18\)
Dấu ''='' xảy ra khi \(x=y=\frac{1}{2}\)
NV
Nguyễn Việt Lâm
Giáo viên
16 tháng 3 2022
\(P=\left(2x+\dfrac{1}{x}\right)^2+\left(2y+\dfrac{1}{y}\right)^2\ge\dfrac{1}{2}\left(2x+\dfrac{1}{x}+2y+\dfrac{1}{y}\right)^2\ge\dfrac{1}{2}\left(2x+2y+\dfrac{4}{x+y}\right)^2=18\)
\(P_{min}=18\) khi \(x=y=\dfrac{1}{2}\)
NV
Nguyễn Việt Lâm
Giáo viên
17 tháng 2 2022
\(x+y\le xy\Rightarrow\dfrac{1}{x}+\dfrac{1}{y}\le1\)
\(M=\dfrac{1}{2\left(x^2+y^2\right)+y^2}+\dfrac{1}{2\left(x^2+y^2\right)+x^2}\le\dfrac{1}{4xy+y^2}+\dfrac{1}{4xy+x^2}\)
\(B\le\dfrac{1}{25}\left(\dfrac{4}{xy}+\dfrac{1}{y^2}\right)+\dfrac{1}{25}\left(\dfrac{4}{xy}+\dfrac{1}{x^2}\right)=\dfrac{1}{25}\left(\dfrac{1}{x^2}+\dfrac{1}{y^2}+\dfrac{2}{xy}+\dfrac{6}{xy}\right)\)
\(M\le\dfrac{1}{25}\left[\left(\dfrac{1}{x}+\dfrac{1}{y}\right)^2+\dfrac{3}{2}\left(\dfrac{1}{x}+\dfrac{1}{y}\right)^2\right]=\dfrac{1}{10}\left(\dfrac{1}{x}+\dfrac{1}{y}\right)^2\le\dfrac{1}{10}\)
\(M_{max}=\dfrac{1}{10}\) khi \(x=y=2\)
NV
Nguyễn Việt Lâm
Giáo viên
18 tháng 2 2022
Sử dụng BĐT cộng mẫu:
\(\dfrac{1}{xy}+\dfrac{1}{xy}+\dfrac{1}{xy}+\dfrac{1}{xy}+\dfrac{1}{y^2}\ge\dfrac{\left(1+1+1+1+1\right)^2}{xy+xy+xy+xy+y^2}=\dfrac{25}{4xy+y^2}\)
\(\Rightarrow\dfrac{1}{4xy+y^2}\le\dfrac{1}{25}\left(\dfrac{4}{xy}+\dfrac{1}{y^2}\right)\)
11 tháng 2 2022
b, Ta có : \(\dfrac{x}{3}=\dfrac{y}{4};\dfrac{y}{5}=\dfrac{z}{6}\Rightarrow\dfrac{x}{15}=\dfrac{y}{20}=\dfrac{z}{24}\)
Đặt \(x=15k;y=20k;z=24k\)
Thay vào A ta được : \(A=\dfrac{30k+60k+96k}{45k+80k+120k}=\dfrac{186k}{245k}=\dfrac{186}{245}\)
NV
Nguyễn Việt Lâm
Giáo viên
3 tháng 3 2021
\(P=\dfrac{x+2y}{2xy}+\dfrac{1}{x+2y}=\dfrac{x+2y}{4}+\dfrac{1}{x+2y}\)
\(P=\dfrac{x+2y}{16}+\dfrac{1}{x+2y}+\dfrac{3\left(x+2y\right)}{16}\)
\(P\ge2\sqrt{\dfrac{x+2y}{16\left(x+2y\right)}}+\dfrac{3}{16}.2\sqrt{2xy}=\dfrac{5}{4}\)
\(P_{min}=\dfrac{5}{4}\) khi \(\left(x;y\right)=\left(2;1\right)\)
HN
4 tháng 5 2017
Ta có: \(\dfrac{16}{2x+y+z}\le\dfrac{1}{x}+\dfrac{1}{x}+\dfrac{1}{y}+\dfrac{1}{z}\)
\(\Leftrightarrow\dfrac{1}{2x+y+z}\le\dfrac{1}{16}\left(\dfrac{2}{x}+\dfrac{1}{y}+\dfrac{1}{z}\right)\left(1\right)\)
Tương tự ta có: \(\left\{{}\begin{matrix}\dfrac{1}{x+2y+z}\le\dfrac{1}{16}\left(\dfrac{1}{x}+\dfrac{2}{y}+\dfrac{1}{z}\right)\left(2\right)\\\dfrac{1}{x+y+2z}\le\dfrac{1}{16}\left(\dfrac{1}{x}+\dfrac{1}{y}+\dfrac{2}{z}\right)\left(3\right)\end{matrix}\right.\)
Cộng (1), (2), (3) vế theo vế ta được:
\(\dfrac{1}{2x+y+z}+\dfrac{1}{x+2y+z}+\dfrac{1}{x+y+2z}\le\dfrac{4}{16}\left(\dfrac{1}{x}+\dfrac{1}{y}+\dfrac{1}{z}\right)=\dfrac{4.4}{16}=1\)
Dấu = xảy ra khi \(x=y=z=\dfrac{3}{4}\)
\(\)\(\dfrac{1}{x}+\dfrac{1}{y}\ge\dfrac{4}{x+y}\Rightarrow2\ge\dfrac{4}{x+y}\Leftrightarrow x+y\ge2\)(chắc bài cho x,y>0?
\(\dfrac{1}{x}+\dfrac{1}{y}\ge\dfrac{2}{\sqrt{xy}}\Rightarrow\sqrt{xy}\ge1\Leftrightarrow xy\ge1\)
\(D=\dfrac{1}{x+2y}+\dfrac{1}{2x+y}=\dfrac{1}{x+y+y}+\dfrac{1}{x+x+y}\le\dfrac{1}{y+2}+\dfrac{1}{x+2}\)
\(cm:\dfrac{1}{x+2}+\dfrac{1}{y+2}\le\dfrac{2}{3}\Leftrightarrow\dfrac{x+y+4}{\left(x+2\right)\left(y+2\right)}\le\dfrac{2}{3}\)
\(\Leftrightarrow2\left(x+2\right)\left(y+2\right)\ge3\left(x+y+4\right)\Leftrightarrow4x+4y+8+2xy\ge3x+3y+12\Leftrightarrow x+y+2xy\ge4\left(1\right)\)
\(x+y\ge2;xy\ge1\Rightarrow\left(1\right)đúng\Rightarrow D\le\dfrac{2}{3}\Rightarrow dấu"="xayra\Leftrightarrow x=y=1\)