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đây là bài tổng quát nè bạn, áp dụng bài này nhé ^_^
https://olm.vn/hoi-dap/question/1123004.html
![](https://rs.olm.vn/images/avt/0.png?1311)
1/ Ta có: \(\frac{x^4}{1a}+\frac{y^4}{b}=\frac{\left(x^2+y^2\right)^2}{a+b}\)
\(\Leftrightarrow1bx^4\left(a+b\right)+ay^4\left(a+b\right)=ab\left(x^4+2x^2y^2+y^4\right)\)
\(\Leftrightarrow\left(ay^2-bx^2\right)^2=0\)
\(\Rightarrow\frac{x^2}{1a}=\frac{y^2}{b}=\frac{\left(x^2+y^2\right)}{a+b}=\frac{1}{a+b}\)
\(\Rightarrow\frac{x^{2006}}{1a^{1003}}=\frac{y^{2006}}{b^{1003}}=\frac{1}{\left(a+b\right)^{1003}}\)
\(\Rightarrow\frac{x^{2006}}{a^{1003}}+\frac{y^{2006}}{b^{1003}}=\frac{2}{\left(a+b\right)^{1003}}\)
![](https://rs.olm.vn/images/avt/0.png?1311)
Nhân cả 2 vế với a+b+c
Chứng minh \(\frac{a}{b}+\frac{b}{a}\ge2\) tương tự với \(\frac{b}{c}+\frac{c}{b};\frac{c}{a}+\frac{a}{c}\)
\(\Leftrightarrow\frac{a}{b}+\frac{b}{a}-2\ge0\Leftrightarrow\frac{a^2-2ab+b^2}{ab}\ge0\Leftrightarrow\frac{\left(a-b\right)^2}{ab}\ge0\)luôn đúng do a;b>0
dễ rồi nhé
b) \(P=\frac{x}{x+1}+\frac{y}{y+1}+\frac{z}{z+1}\)
\(P=\left(\frac{x+1}{x+1}+\frac{y+1}{y+1}+\frac{z+1}{z+1}\right)-\left(\frac{1}{x+1}+\frac{1}{y+1}+\frac{1}{z+1}\right)\)
\(P=\left(1+1+1\right)-\left(\frac{1}{x+1}+\frac{1}{y+1}+\frac{1}{z+1}\right)\)
\(P=3-\left(\frac{1}{x+1}+\frac{1}{y+1}+\frac{1}{z+1}\right)\)
Áp dụng bđt Cauchy Schwarz dạng Engel (mình nói bđt như vậy,chỗ này bạn cứ nói theo cái bđt đề bài cho đi) ta được:
\(\frac{1}{x+1}+\frac{1}{y+1}+\frac{1}{z+1}\ge\frac{\left(1+1+1\right)^2}{x+1+y+1+z+1}=\frac{9}{4}\)
=>\(P=3-\left(\frac{1}{x+1}+\frac{1}{y+1}+\frac{1}{z+1}\right)\le3-\frac{9}{4}=\frac{3}{4}\)
=>Pmax=3/4 <=> x=y=z=1/3
![](https://rs.olm.vn/images/avt/0.png?1311)
Ta co:
\(\frac{x^4}{a}+\frac{y^4}{b}\ge\frac{\left(x^2+y^2\right)^2}{a+b}=\frac{1}{a+b}\)
Dau '=' xay ra khi \(\frac{x^2}{a}=\frac{y^2}{b}\)
Ta lai co:
\(\frac{x^6}{a^3}+\frac{y^6}{b^3}=\left(\frac{x^2}{a}\right)^3+\left(\frac{y^2}{b}\right)^3=2\left(\frac{x^2}{a}\right)^3\)
Ma \(\frac{x^2}{a}=\frac{y^2}{b}=\frac{x^2+y^2}{a+b}=\frac{1}{a+b}\)
\(\Rightarrow x^2=\frac{a}{a+b}\)
\(\Leftrightarrow\frac{x^2}{a}=\frac{1}{a+b}\)
\(\Leftrightarrow\left(\frac{x^2}{a}\right)^3=\frac{1}{\left(a+b\right)^3}\)
\(\Rightarrow\frac{x^6}{a^3}+\frac{y^6}{b^3}=\frac{2}{\left(a+b\right)^3}\)
![](https://rs.olm.vn/images/avt/0.png?1311)
Đặt \(\left(\frac{a-b}{c},\frac{b-c}{a},\frac{c-a}{b}\right)\rightarrow\left(x,y,z\right)\)
Khi đó:\(\left(\frac{c}{a-b},\frac{a}{b-c},\frac{b}{c-a}\right)\rightarrow\left(\frac{1}{x},\frac{1}{y},\frac{1}{z}\right)\)
Ta có:
\(P\cdot Q=\left(x+y+z\right)\left(\frac{1}{x}+\frac{1}{y}+\frac{1}{z}\right)=3+\frac{y+z}{x}+\frac{z+x}{y}+\frac{x+y}{z}\)
Mặt khác:\(\frac{y+z}{x}=\left(\frac{b-c}{a}+\frac{c-a}{b}\right)\cdot\frac{c}{a-b}=\frac{b^2-bc+ac-a^2}{ab}\cdot\frac{c}{a-b}\)
\(=\frac{c\left(a-b\right)\left(c-a-b\right)}{ab\left(a-b\right)}=\frac{c\left(c-a-b\right)}{ab}=\frac{2c^2}{ab}\left(1\right)\)
Tương tự:\(\frac{x+z}{y}=\frac{2a^2}{bc}\left(2\right)\)
\(=\frac{x+y}{z}=\frac{2b^2}{ac}\left(3\right)\)
Từ ( 1 );( 2 );( 3 ) ta có:
\(P\cdot Q=3+\frac{2c^2}{ab}+\frac{2a^2}{bc}+\frac{2b^2}{ac}=3+\frac{2}{abc}\left(a^3+b^3+c^3\right)\)
Ta có:\(a+b+c=0\)
\(\Rightarrow\left(a+b\right)^3=-c^3\)
\(\Rightarrow a^3+b^3+3ab\left(a+b\right)=-c^3\)
\(\Rightarrow a^3+b^3+c^3=3abc\)
Khi đó:\(P\cdot Q=3+\frac{2}{abc}\cdot3abc=9\)
\(\Leftrightarrow\frac{x^4}{a}+\frac{y^4}{b}=\frac{\left(x^2+y^2\right)^2}{a+b}\)
\(\Leftrightarrow\frac{x^4b+y^4a}{ab}=\frac{x^4+y^4+2x^2y^2}{a+b}\Leftrightarrow\left(a+b\right)\left(x^4b+y^4a\right)=ab\left(x^4+y^4+2x^2y^2\right)\)
\(\Leftrightarrow x^4ab+y^4a^2+x^4b^2+y^4ab=x^4ab+y^4ab+2x^2y^2ab\)
\(\Leftrightarrow y^4a^2+x^4b^2=2x^2y^2ab\Leftrightarrow\left(x^2b-y^2a\right)^2=0\Leftrightarrow\frac{x^2}{a}=\frac{y^2}{b}\)
\(\Rightarrow\left(\frac{x^2}{a}\right)^{1001}=\left(\frac{y^2}{b}\right)^{1001}\Leftrightarrow\frac{x^{2002}}{a^{1001}}=\frac{y^{2002}}{b^{2011}}\)
Mà: \(\frac{x^2}{a}=\frac{y^2}{b}=\frac{x^2+y^2}{a+b}=\frac{1}{a+b}\Leftrightarrow\left(\frac{x^2}{a}\right)^{1001}=\frac{1}{\left(a+b\right)^{1001}}\)
\(\Rightarrow\frac{x^{2002}}{a^{1001}}+\frac{y^{2002}}{b^{1001}}=\frac{2}{\left(a+b\right)^{1001}}\left(đpcm\right)\)
\(x^2+y^2=1\Rightarrow y^2=1-x^2\)
\(\frac{x^4}{a}+\frac{y^4}{b}=\frac{1}{a+b}\Leftrightarrow\frac{b.x^4+a.y^4}{ab}=\frac{1}{a+b}\)
\(\Leftrightarrow bx^4+ay^4=\frac{ab}{a+b}\Leftrightarrow bx^4+a\left(1-x^2\right)^2-\frac{ab}{a+b}=0\)
\(\Leftrightarrow bx^4+a\left(x^4-2x^2+1\right)-\frac{ab}{a+b}=0\)
\(\Leftrightarrow\left(a+b\right)x^4-2ax^2+a-\frac{ab}{a+b}=0\)
\(\Leftrightarrow\left(a+b\right)x^4-2ax^2+\frac{a^2}{a+b}=0\Leftrightarrow\left(a+b\right)\left[x^4-2.x.\frac{a}{a+b}+\left(\frac{a}{a+b}\right)^2\right]=0\)
\(\Leftrightarrow\left(a+b\right)\left(x^2-\frac{a}{a+b}\right)=0\Rightarrow x^2=\frac{a}{a+b}\) (do \(a+b\ne0\))
\(\Rightarrow y^2=1-x^2=\frac{b}{a+b}\)
\(\Rightarrow\) \(\frac{x^2}{a}=\frac{a}{a\left(a+b\right)}=\frac{1}{a+b}\) ; \(\frac{y^2}{b}=\frac{b}{b\left(a+b\right)}=\frac{1}{a+b}\)
Thay vào bài toán:
\(\frac{x^{2002}}{a^{1001}}+\frac{y^{2002}}{b^{1001}}=\left(\frac{x^2}{a}\right)^{1001}+\left(\frac{y^2}{b}\right)^{1001}=\left(\frac{1}{a+b}\right)^{1001}+\left(\frac{1}{a+b}\right)^{1001}=\frac{2}{\left(a+b\right)^{1001}}\)