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Bài lớp 8 thật hả? :(
\(\frac{a}{a+b+1}+\frac{b}{b+c+1}+\frac{c}{c+a+1}\le1\)
\(\Leftrightarrow\frac{a}{4-a}+\frac{b}{4-b}+\frac{c}{4-c}\le1\)
\(\Leftrightarrow a\left(4-b\right)\left(4-c\right)+b\left(4-a\right)\left(4-c\right)+c\left(4-a\right)\left(4-b\right)\le\left(4-a\right)\left(4-b\right)\left(4-c\right)\)
\(\Leftrightarrow a^2b+ac^2+b^2c+abc\le4\) (1)
Ta cần chứng minh (1)
Không mất tính tổng quát, giả sử \(a\le c\le b\)
\(\Rightarrow a\left(a-c\right)\left(b-c\right)\le0\)
\(\Leftrightarrow a^2b+ac^2\le a^2c+abc\)
\(\Leftrightarrow a^2b+ac^2+b^2c+abc\le a^2c+abc+b^2c+abc\)
\(\Leftrightarrow a^2b+ac^2+b^2c+abc\le c\left(a+b\right)^2\)
\(\Leftrightarrow a^2b+ac^2+b^2c+abc\le\frac{1}{2}.2c\left(a+b\right)\left(a+b\right)\le\frac{1}{2}.\frac{\left(2c+a+b+a+b\right)^3}{27}\)
\(\Leftrightarrow a^2b+ac^2+b^2c+abc\le\frac{1}{2}.\frac{8.3^3}{27}=4\) (đpcm)
Dấu "=" xảy ra khi \(a=b=c=1\)
Do biểu thức đề bài và BĐT đều mang tính đối xứng, không mất tính tổng quát giả sử \(a\ge b\ge c\)
Đặt \(\left(x;y;z\right)=\left(b+c-a;c+a-b;a+b-c\right)\) \(\Rightarrow\left\{{}\begin{matrix}y>0\\z>0\end{matrix}\right.\)
Ta cần chứng minh \(xyz\le1\)
Nếu \(x\le0\) thì \(xyz\le0\Rightarrow xyz< 1\) BĐT hiển nhiên đúng
Nếu \(x>0\)
\(\Rightarrow\left\{{}\begin{matrix}a=\frac{y+z}{2}\\b=\frac{x+z}{2}\\c=\frac{x+y}{2}\end{matrix}\right.\) \(\Rightarrow x+y+z=\frac{2}{x+y}+\frac{2}{y+z}+\frac{2}{z+x}\)
\(\Rightarrow x+y+z\le\frac{1}{\sqrt{xy}}+\frac{1}{\sqrt{yz}}+\frac{1}{\sqrt{zx}}\)
\(\Leftrightarrow\sqrt{xyz}\left(x+y+z\right)\le\sqrt{x}+\sqrt{y}+\sqrt{z}\)
\(\Leftrightarrow xyz\left(x+y+z\right)^2\le\left(\sqrt{x}+\sqrt{y}+\sqrt{z}\right)^2\le3\left(x+y+z\right)\)
\(\Leftrightarrow xyz\left(x+y+z\right)\le3\)
\(\Leftrightarrow xyz.3\sqrt[3]{xyz}\le xyz\left(x+y+z\right)\le3\)
\(\Leftrightarrow xyz\sqrt[3]{xyz}\le1\Leftrightarrow xyz\le1\) (đpcm)
Dấu "=" xảy ra khi \(x=y=z=1\) hay \(a=b=c=1\)
\(\frac{a+bc}{b+c}+\frac{b+ac}{c+a}+\frac{c+ab}{a+b}\)
\(=\frac{a\left(a+b+c\right)+bc}{b+c}+\frac{b\left(a+b+c\right)+ac}{a+c}+\frac{c\left(a+b+c\right)+ab}{a+b}\)
\(=\frac{\left(a+b\right)\left(a+c\right)}{b+c}+\frac{\left(a+b\right)\left(b+c\right)}{a+c}+\frac{\left(c+a\right)\left(c+b\right)}{a+b}\)
Áp dụng bđt Cô Si: \(\frac{\left(a+b\right)\left(a+c\right)}{b+c}+\frac{\left(a+b\right)\left(b+c\right)}{a+c}\ge2\left(a+b\right)\)
Tương tự,cộng theo vế và rút gọn =>đpcm
\(\frac{a+bc}{b+c}+\frac{b+ac}{c+a}+\frac{c+ab}{a+b}\)
\(=\frac{a\left(a+b+c\right)+bc}{b+c}+\frac{b\left(a+b+c\right)+ac}{a+c}+\frac{c\left(a+b+c\right)+ab}{a+b}\)
\(=\frac{\left(a+b\right)\left(a+c\right)}{b+c}+\frac{\left(a+b\right)\left(b+c\right)}{a+c}+\frac{\left(c+a\right)\left(c+b\right)}{a+b}\)
Áp dụng bđt CÔ si
\(\frac{\left(a+b\right)\left(a+c\right)}{b+c}+\frac{\left(a+b\right)\left(b+c\right)}{a+c}\ge2\left(a+b\right)\)
.............
Thực hiện phép biến đổi tương đương:
\(\Leftrightarrow\frac{a^2+b^2+2}{\left(1+a^2\right)\left(1+b^2\right)}\ge\frac{2}{1+ab}\)
\(\Leftrightarrow\left(1+ab\right)\left(a^2+b^2+2\right)\ge2\left(1+a^2+b^2+a^2b^2\right)\)
\(\Leftrightarrow a^2+b^2+2+a^3b+ab^3+2ab\ge2+2a^2+2b^2+2a^2b^2\)
\(\Leftrightarrow a^3b-2a^2b^2+ab^3-a^2+2ab-b^2\ge0\)
\(\Leftrightarrow ab\left(a-b\right)^2-\left(a-b\right)^2\ge0\)
\(\Leftrightarrow\left(ab-1\right)\left(a-b\right)^2\ge0\) (luôn đúng do \(ab>1\))
Dấu "=" xảy ra khi \(a=b\)
Áp dụng bđt Cauchy-Schwarz :
\(\frac{a^2}{b+c}+\frac{b^2}{a+c}+\frac{c^2}{a+b}\ge\frac{\left(a+b+c\right)^2}{2\left(a+b+c\right)}=\frac{a+b+c}{2}\)
Đề thiếu không bạn ?
\(8VT=4\left(a^2b+b^2c+c^2a+abc\right)\left(2ab^2+2bc^2+2ca^2+2abc\right)\le\left(a^2b+b^2c+c^2a+2ab^2+2bc^2+2ca^2+3abc\right)^2\)
\(\Rightarrow VT\le\frac{1}{32}\left(2a^2b+2b^2c+2c^2a+4ca^2+4ab^2+4bc^2+6abc\right)^2\)
\(\Rightarrow VT\le\frac{1}{32}\left(2a^2b+2b^2c+2c^2a+4ca^2+4ab^2+4bc^2+9abc\right)^2\)
\(\Rightarrow VT\le\frac{1}{32}\left[\left(a+2b\right)\left(b+2c\right)\left(c+2a\right)\right]^2\)
\(\Rightarrow VT\le\frac{1}{512}\left[\left(a+2b\right)\left(4b+8c\right)\left(c+2a\right)\right]^2\)
\(\Rightarrow VT\le\frac{1}{512}\left(\frac{a+2b+4b+8c+c+2a}{3}\right)^6=\frac{1}{512}\left(a+2b+3c\right)^6=\frac{4^6}{512}=8\)
Dấu "=" xảy ra khi \(\left(a;b;c\right)=\left(2;1;0\right)\)
Nhân 2 vế của 2 ĐT đề bài ta có
\(\left(a+b+c\right)\left(\frac{1}{a+b}+\frac{1}{b+c}+\frac{1}{a+c}\right)=\frac{47}{10}\)
<=> \(\frac{a}{b+c}+\frac{b}{a+c}+\frac{c}{a+b}+\left(\frac{a}{a+b}+\frac{b}{a+b}\right)+\left(\frac{b}{b+c}+\frac{c}{b+c}\right)+\left(\frac{c}{a+c}+\frac{a}{a+c}\right)=\frac{47}{10}\)
=>\(P=\frac{17}{10}\)
Vậy \(P=\frac{17}{10}\)
\(ab+bc+ca=2abc\Leftrightarrow\frac{1}{a}+\frac{1}{b}+\frac{1}{c}=2\)
Đặt \(\left(\frac{1}{a};\frac{1}{b};\frac{1}{c}\right)=\left(x;y;z\right)\)
\(P=\frac{x^3}{\left(2-x\right)^2}+\frac{y^3}{\left(2-y\right)^2}+\frac{z^3}{\left(2-z\right)^2}\)
Ta có đánh giá: \(\frac{x^3}{\left(2-x\right)^2}\ge\frac{2x-1}{2}\) \(\forall x:0< x< 2\)
\(\Leftrightarrow2x^3\ge\left(2x-1\right)\left(2-x\right)^2\)
\(\Leftrightarrow9x^2-12x+4\ge0\)
\(\Leftrightarrow\left(3x-2\right)^2\ge0\) (luôn đúng)
Tương tự: \(\frac{y^3}{\left(2-y\right)^2}\ge\frac{2y-1}{2}\) ; \(\frac{z^3}{\left(2-z\right)^2}\ge\frac{2z-1}{2}\)
Cộng vế với vế: \(P\ge\frac{2\left(x+y+z\right)-3}{2}=\frac{4-3}{2}=\frac{1}{2}\)
Dấu "=" xảy ra khi \(x=y=z=\frac{2}{3}\) hay \(a=b=c=\frac{3}{2}\)
Có: \(VT=\frac{\left(a+c\right)\left(b+c\right)}{a+b}+\frac{\left(a+b\right)\left(c+a\right)}{b+c}+\frac{\left(c+b\right)\left(a+b\right)}{a+c}\) (thay a+ b+c=1 vào r phân tích thành nhân tử)
Lại có: Theo Cô si \(\frac{\left(a+c\right)\left(b+c\right)}{a+b}+\frac{\left(a+b\right)\left(c+a\right)}{b+c}\ge2\left(c+a\right)\)
Tương tự với hai BĐT còn lại và cộng theo vế được: \(2VT\ge4\Leftrightarrow VT\ge2^{\left(đpcm\right)}\)
"=" <=> a = b = c = 1/3
Đặt \(P=\frac{ab+c}{a+b}+\frac{bc+a}{b+c}+\frac{ac+b}{a+c}=\frac{ab+c\left(a+b+c\right)}{a+b}+\frac{bc+a\left(a+b+c\right)}{b+c}+\frac{ac+b\left(a+b+c\right)}{a+c}\)
\(=\frac{\left(a+c\right)\left(b+c\right)}{a+b}+\frac{\left(a+b\right)\left(a+c\right)}{b+c}+\frac{\left(a+b\right)\left(b+c\right)}{a+c}\)
Ta có:
\(\frac{\left(a+c\right)\left(b+c\right)}{a+b}+\frac{\left(a+b\right)\left(a+c\right)}{b+c}\ge2\left(a+c\right)\)
\(\frac{\left(a+b\right)\left(a+c\right)}{b+c}+\frac{\left(a+b\right)\left(b+c\right)}{a+c}\ge2\left(a+b\right)\)
\(\frac{\left(a+c\right)\left(b+c\right)}{a+b}+\frac{\left(a+b\right)\left(b+c\right)}{a+c}\ge2\left(b+c\right)\)
Cộng vế với vế
\(2P\ge4\left(a+b+c\right)=4\Rightarrow P\ge2\)
Dấu "=" xảy ra khi \(a=b=c=\frac{1}{3}\)