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![](https://rs.olm.vn/images/avt/0.png?1311)
\(\sqrt{\dfrac{ab}{c+ab}}=\sqrt{\dfrac{ab}{c\left(a+b+c\right)+ab}}=\sqrt{\dfrac{ab}{\left(a+c\right)\left(b+c\right)}}\le\dfrac{1}{2}\left(\dfrac{a}{a+c}+\dfrac{b}{b+c}\right)\)
Tương tự: \(\sqrt{\dfrac{bc}{a+bc}}\le\dfrac{1}{2}\left(\dfrac{b}{a+b}+\dfrac{c}{a+c}\right)\) ; \(\sqrt{\dfrac{ca}{b+ca}}\le\dfrac{1}{2}\left(\dfrac{a}{a+b}+\dfrac{c}{b+c}\right)\)
Cộng vế với vế:
\(P\le\dfrac{1}{2}\left(\dfrac{a}{a+c}+\dfrac{c}{a+c}+\dfrac{b}{b+c}+\dfrac{c}{b+c}+\dfrac{b}{a+b}+\dfrac{a}{a+b}\right)=\dfrac{3}{2}\)
Dấu "=" xảy ra khi \(a=b=c=\dfrac{1}{3}\)
Cho a, b, c, d là các chữ số thỏa mãn: ab+ca=da ab-ca=a Tìm giá trị của d.
![](https://rs.olm.vn/images/avt/0.png?1311)
ta có \(\frac{a}{\sqrt{a+bc}}=\frac{a}{\sqrt{a\left(a+b+c\right)+bc}}=\frac{a}{\sqrt{\left(a+b\right)\left(a+c\right)}}\le\frac{1}{2}\left(\frac{a}{a+b}+\frac{a}{a+c}\right)\)
Tương tự rồi cộng lại = P<=3/2
dâu = xảy ra <=> a=b=c=1/3
^^
Xét \(\frac{a}{\sqrt{a+bc}}=\sqrt{\frac{a^2}{a+bc}}\)
Ta có: a + bc = 1-b-c+bc ( Do a=1-b-c ) => a+bc = 1-b-c+bc = (b-1)(c-1)
=> \(\sqrt{\frac{a^2}{a+bc}}=\sqrt{\frac{a^2}{1-b-c+bc}}=\sqrt{\frac{a^2}{\left(b-1\right)\left(c-1\right)}}=\sqrt{\frac{a}{b-1}.\frac{a}{c-1}}\le\frac{1}{2}\left(\frac{a}{b-1}+\frac{b}{c-1}\right)\)
![](https://rs.olm.vn/images/avt/0.png?1311)
\(a^2+b^2-ab\ge\dfrac{1}{2}\left(a+b\right)^2-\dfrac{1}{4}\left(a+b\right)^2=\dfrac{1}{4}\left(a+b\right)^2\)
\(\Rightarrow\dfrac{1}{\sqrt{a^2-ab+b^2}}\le\dfrac{1}{\sqrt{\dfrac{1}{4}\left(a+b\right)^2}}=\dfrac{2}{a+b}\le\dfrac{1}{2}\left(\dfrac{1}{a}+\dfrac{1}{b}\right)\)
Tương tự:
\(\dfrac{1}{\sqrt{b^2-bc+c^2}}\le\dfrac{1}{2}\left(\dfrac{1}{b}+\dfrac{1}{c}\right)\) ; \(\dfrac{1}{\sqrt{c^2-ca+a^2}}\le\dfrac{1}{2}\left(\dfrac{1}{c}+\dfrac{1}{a}\right)\)
Cộng vế:
\(P\le\dfrac{1}{a}+\dfrac{1}{b}+\dfrac{1}{c}=3\)
Dấu "=" xảy ra khi \(a=b=c=1\)
![](https://rs.olm.vn/images/avt/0.png?1311)
Ta có:
sigma \(\frac{ab}{3a+4b+5c}=\) sigma \(\frac{2ab}{5\left(a+b+2c\right)+\left(a+3b\right)}\le\frac{2}{36}\left(sigma\frac{5ab}{a+b+2c}+sigma\frac{ab}{a+3b}\right)\)
Ta đi chứng minh: \(sigma\frac{ab}{a+b+2c}\le\frac{9}{4}\)
có: \(sigma\frac{ab}{a+b+2c}\le\frac{1}{4}\left(sigma\frac{ab}{c+a}+sigma\frac{ab}{b+c}\right)=\frac{1}{4}\left(a+b+c\right)=\frac{9}{4}\)
BĐT trên đúng nếu: \(sigma\frac{ab}{a+3b}\le\frac{9}{4}\)
Ta thấy: \(sigma\frac{ab}{a+3b}\le\frac{1}{16}\left(sigma\frac{ab}{a}+sigma\frac{3ab}{b}\right)=\frac{1}{16}\)( sigma \(b+sigma3a\)) \(=\frac{1}{4}\left(a+b+c\right)=\frac{9}{4}\)
\(\Leftrightarrow sigma\frac{ab}{3a+4b+5c}\le\frac{1}{18}\left(5.\frac{9}{4}+\frac{9}{4}\right)=\frac{3}{4}\)(1)
MÀ: \(\frac{1}{\sqrt{ab\left(a+2c\right)\left(b+2c\right)}}=\frac{2}{2\sqrt{\left(ab+2bc\right)\left(ab+2ca\right)}}\ge\frac{2}{2\left(ab+bc+ca\right)}\)
\(=\frac{3}{3\left(ab+bc+ca\right)}\ge\frac{3}{\left(a+b+c\right)^2}=\frac{3}{9^2}=\frac{1}{27}\)(2)
Từ (1) và (2) \(\Rightarrow T\le\frac{3}{4}-\frac{1}{27}=\frac{77}{108}\)
Vậy GTLN của biểu thức T là 77/108 <=> a=b=c=3
\(P^2=\left(\sqrt{4a+3}+\sqrt{4b+3}+\sqrt{4c+3}\right)^2\)
\(\le\left(1^2+1^2+1^2\right)\left(4a+3+4b+3+3c+3\right)\)
\(=63\)
\(\Rightarrow P\le\sqrt{63}=3\sqrt{7}\).
Dấu \(=\)khi \(\hept{\begin{cases}4a+3=4b+3=4c+3\\a+b+c=3\end{cases}}\Leftrightarrow a=b=c=1\).