Tìm a, b, c thỏa mãn:
\(\hept{\begin{cases}a^4-2b=\frac{-1}{2}\\b^4-2c=\frac{-1}{2}\\c^4-2a=\frac{-1}{2}\end{cases}}\)
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18 tháng 11 2019
Bài 2:
\(\frac{1}{\sqrt[3]{81}}\cdot P=\frac{1}{\sqrt[3]{9\cdot9\cdot\left(a+2b\right)}}+\frac{1}{\sqrt[3]{9\cdot9\cdot\left(b+2c\right)}}+\frac{1}{\sqrt[3]{9\cdot9\cdot\left(c+2a\right)}}\)
\(\ge\frac{3}{a+2b+9+9}+\frac{3}{b+2c+9+9}+\frac{3}{c+2a+9+9}\ge3\left(\frac{9}{3a+3b+3c+54}\right)=\frac{1}{3}\)
\(\Rightarrow P\ge\sqrt[3]{3}\)
Dấu bằng xẩy ra khi a=b=c=3
18 tháng 11 2019
Bài 1:
\(ab+bc+ca=5abc\Rightarrow\frac{1}{a}+\frac{1}{b}+\frac{1}{c}=5\)
Theo bđt côsi-shaw ta luôn có: \(\frac{1}{x}+\frac{1}{y}+\frac{1}{z}+\frac{1}{t}+\frac{1}{k}\ge\frac{25}{x+y+z+t+k}\)(x=y=z=t=k>0 ) (*)
\(\Leftrightarrow\left(x+y+z+t+k\right)\left(\frac{1}{x}+\frac{1}{y}+\frac{1}{z}+\frac{1}{t}+\frac{1}{k}\right)\ge25\)
Áp dụng bđt AM-GM ta có:
\(\hept{\begin{cases}x+y+z+t+k\ge5\sqrt[5]{xyztk}\\\frac{1}{x}+\frac{1}{y}+\frac{1}{z}+\frac{1}{t}+\frac{1}{k}\ge5\sqrt[5]{\frac{1}{xyztk}}\end{cases}}\)
\(\Rightarrow\left(x+y+z+t+k\right)\left(\frac{1}{x}+\frac{1}{y}+\frac{1}{z}+\frac{1}{t}+\frac{1}{k}\right)\ge25\)
\(\Rightarrow\)(*) luôn đúng
Từ (*) \(\Rightarrow\frac{1}{25}\left(\frac{1}{x}+\frac{1}{y}+\frac{1}{z}+\frac{1}{t}+\frac{1}{k}\right)\le\frac{1}{x+y+z+t+k}\)
Ta có: \(P=\frac{1}{2a+2b+c}+\frac{1}{a+2b+2c}+\frac{1}{2a+b+2c}\)
Mà \(\frac{1}{2a+2b+c}=\frac{1}{a+a+b+b+c}\le\frac{1}{25}\left(\frac{1}{a}+\frac{1}{a}+\frac{1}{b}+\frac{1}{b}+\frac{1}{c}\right)\)
\(\frac{1}{a+2b+2c}=\frac{1}{a+b+b+c+c}\le\frac{1}{25}\left(\frac{1}{a}+\frac{1}{b}+\frac{1}{b}+\frac{1}{c}+\frac{1}{c}\right)\)
\(\frac{1}{2a+b+2c}=\frac{1}{a+a+b+c+c}\le\frac{1}{25}\left(\frac{1}{a}+\frac{1}{a}+\frac{1}{b}+\frac{1}{c}+\frac{1}{c}\right)\)
\(\Rightarrow P\le\frac{1}{25}\left[5.\left(\frac{1}{a}+\frac{1}{b}+\frac{1}{c}\right)\right]=1\)
\(\Rightarrow P\le1\left(đpcm\right)\)Dấu"="xảy ra khi a=b=c\(=\frac{3}{5}\)
29 tháng 6 2017
Ta có:
\(\left(a+b+c+d\right)\left(\frac{1}{a}+\frac{1}{b}+\frac{1}{c}+\frac{1}{d}\right)\ge\left(a+b+c+d\right).\frac{16}{\left(a+b+c+d\right)}=16\)
\(\Rightarrow\frac{1}{a}+\frac{1}{b}+\frac{1}{c}+\frac{1}{d}\ge4\)
Dấu = xảy ra khi \(a=b=c=d=1\)
17 tháng 6 2019
Áp dụng bđt Cô-si có'
\(\frac{1}{x}+\frac{1}{y}\ge\frac{2}{\sqrt{xy}}\ge\frac{2}{\frac{x+y}{2}}=\frac{4}{x+y}\)
\(\Rightarrow\frac{1}{x}+\frac{1}{y}\ge\frac{4}{x+y}\)
\(\Rightarrow\frac{1}{x+y}\le\frac{1}{4}\left(\frac{1}{x}+\frac{1}{y}\right)\)(1)
Áp dụng bđt trên ta được
\(\frac{1}{2a+b+c}=\frac{1}{\left(a+b\right)+\left(a+c\right)}\le\frac{1}{4}\left(\frac{1}{a+b}+\frac{1}{a+c}\right)\)
\(\Rightarrow\left(\frac{1}{2a+b+c}\right)^2\le\frac{1}{16}\left(\frac{1}{a+b}+\frac{1}{a+c}\right)^2\)
Chứng minh tương tự rồi cộng các vế lại cho nhau ta được
\(A\le\frac{1}{16}\left(\frac{1}{a+b}+\frac{1}{a+c}\right)^2+\frac{1}{16}\left(\frac{1}{a+c}+\frac{1}{b+c}\right)^2+\frac{1}{16}\left(\frac{1}{a+b}+\frac{1}{b+c}\right)^2\)
\(\Rightarrow16A\le\left(\frac{1}{a+b}+\frac{1}{a+c}\right)^2+\left(\frac{1}{a+c}+\frac{1}{b+c}\right)^2+\left(\frac{1}{a+b}+\frac{1}{b+c}\right)^2\)
\(=\frac{2}{\left(a+b\right)^2}+\frac{2}{\left(b+c\right)^2}+\frac{2}{\left(c+a\right)^2}+\frac{2}{\left(a+b\right)\left(a+c\right)}+\frac{2}{\left(b+c\right)\left(a+b\right)}+\frac{2}{\left(a+c\right)\left(b+c\right)}\)
Đặt \(\left(\frac{1}{a+b};\frac{1}{b+c};\frac{1}{c+a}\right)\rightarrow\left(x;y;z\right)\)
Khi đó \(16A\le2x^2+2y^2+2z^2+2xy+2yz+2zx\)
Ta có bđt phụ sau : \(xy+yz+zx\le x^2+y^2+z^2\)(tự chứng minh) (2)
Áp dụng ta được
\(16A\le4x^2+4y^2+4z^2=\frac{4}{\left(a+b\right)^2}+\frac{4}{\left(b+c\right)^2}+\frac{4}{\left(c+a\right)^2}\)
\(\Rightarrow4A\le\frac{1}{\left(a+b\right)^2}+\frac{1}{\left(b+c\right)^2}+\frac{1}{\left(c+a\right)^2}\)
Từ (1) \(\Rightarrow\frac{1}{\left(x+y\right)^2}\le\frac{1}{16}\left(\frac{1}{x}+\frac{1}{y}\right)^2\)(Bình phương 2 vế lên)
Áp dụng bđt này ta được
\(4A\le\frac{1}{16}\left(\frac{1}{a}+\frac{1}{b}\right)^2+\frac{1}{16}\left(\frac{1}{b}+\frac{1}{c}\right)^2+\frac{1}{16}\left(\frac{1}{c}+\frac{1}{a}\right)^2\)
\(\Rightarrow64A\le\frac{1}{a^2}+\frac{2}{ab}+\frac{1}{b^2}+\frac{1}{b^2}+\frac{2}{bc}+\frac{1}{c^2}+\frac{1}{c^2}+\frac{2}{ac}+\frac{1}{a^2}\)
\(\Rightarrow64A\le\frac{2}{a^2}+\frac{2}{b^2}+\frac{2}{c^2}+\frac{2}{ab}+\frac{2}{bc}+\frac{2}{ca}\)
\(\Rightarrow32A\le\frac{1}{a^2}+\frac{1}{b^2}+\frac{1}{c^2}+\frac{1}{ab}+\frac{1}{bc}+\frac{1}{ca}\)
Áp dụng bđt (2) ta được \(\frac{1}{ab}+\frac{1}{bc}+\frac{1}{ca}\le\frac{1}{a^2}+\frac{1}{b^2}+\frac{1}{c^2}\)
\(\Rightarrow32A\le\frac{1}{a^2}+\frac{1}{b^2}+\frac{1}{c^2}+\frac{1}{a^2}+\frac{1}{b^2}+\frac{1}{c^2}=3+3=6\)
\(\Rightarrow A\le\frac{6}{32}=\frac{3}{16}\)
Dấu "=" xảy ra tại a=b=c = 1
T
17 tháng 6 2019
#)Em thấy có link này có cách giải ngắn gọn hơn nek :
https://h.vn/hoi-dap/tim-kiem?q=cho+c%C3%A1c+s%E1%BB%91+th%E1%BB%B1c+d%C6%B0%C6%A1ng+a,b,c+thay+%C4%91%E1%BB%95i+lu%C3%B4n+th%E1%BB%8Fa+m%C3%A3n+1/a2+++1/b2+++1/c2+=3.T%C3%ACm+Max+P+=+1/(2a+b+c)2++1(2b+a+c)2++1/(2c+a+b)2&id=394201
Ai cần link này ib e nhé ! e gửi cho chị #Diệp Song Thiên đã ^^
29 tháng 5 2018
4. Ta có: \(a+b+c=6abc\)
\(\Rightarrow\frac{1}{ab}+\frac{1}{bc}+\frac{1}{ca}=6\)
Đặt \(\frac{1}{a}=x;\frac{1}{b}=y;\frac{1}{c}=z\)
\(\Rightarrow xy+yz+zx=6\)
Lại có: \(\frac{bc}{a^3\left(c+2b\right)}=\frac{1}{a^3\frac{c+2b}{bc}}=\frac{\frac{1}{a^3}}{\frac{1}{b}+\frac{2}{c}}=\frac{x^3}{y+2z}\)
Tương tự suy ra:
\(S=\frac{x^3}{y+2z}+\frac{y^3}{z+2x}+\frac{z^3}{x+2y}\)
\(=\frac{x^4}{xy+2zx}+\frac{y^4}{yz+2xy}+\frac{z^4}{zx+2yz}\)
\(\ge\frac{\left(x^2+y^2+z^2\right)^2}{3\left(xy+yz+zx\right)}\ge\frac{x^2+y^2+z^2}{3}\ge\frac{xy+yz+zx}{3}=2\)
Dấu = xảy ra khi \(x=y=z=\sqrt{2}\Rightarrow a=b=c=\frac{1}{\sqrt{2}}\)
