cmr\(\frac{\left(a+b\right)^2}{2}+\frac{a+b}{4}\ge a\sqrt{b}+b\sqrt{a}\left(a,b>0\right)\)
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8 tháng 1 2020
\(\left(a+b\right)\left(b+c\right)\left(c+a\right)+abc\)
\(=abc+a^2b+ab^2+a^2c+ac^2+b^2c+bc^2+abc+abc\)
\(=\left(a+b+c\right)\left(ab+bc+ca\right)\)( phân tích nhân tử các kiểu )
\(\Rightarrow\left(a+b\right)\left(b+c\right)\left(c+a\right)\ge\left(a+b+c\right)\left(ab+bc+ca\right)-abc\left(1\right)\)
\(a+b+c\ge3\sqrt[3]{abc};ab+bc+ca\ge3\sqrt[3]{a^2b^2c^2}\Rightarrow\left(a+b+c\right)\left(ab+bc+ca\right)\ge9abc\)
\(\Rightarrow-abc\ge\frac{-\left(a+b+c\right)\left(ab+bc+ca\right)}{9}\)
Khi đó:\(\left(a+b+c\right)\left(ab+bc+ca\right)-abc\)
\(\ge\left(a+b+c\right)\left(ab+bc+ca\right)-\frac{\left(a+b+c\right)\left(ab+bc+ca\right)}{9}\)
\(=\frac{8\left(a+b+c\right)\left(ab+bc+ca\right)}{9}\left(2\right)\)
Từ ( 1 ) và ( 2 ) có đpcm
QT
25 tháng 6 2021
+) Ta có \(\sqrt{4a\left(3a+b\right)}\le\frac{4a+\left(3a+b\right)}{2}=\frac{7a+b}{2}\)
\(\Rightarrow\sqrt{a\left(3a+b\right)}\le\frac{7a+b}{4}\left(2\right)\)
+) Tương tự ta lại có :
\(\sqrt{b\left(3b+a\right)}\le\frac{7b+a}{4}\left(3\right)\)
+) Từ (2) và (3) ta có :
\(VT\left(1\right)\ge\frac{a+b}{\frac{7a+b}{4}+\frac{7b+a}{4}}=\frac{1}{2}\left(đpcm\right)\)
25 tháng 6 2021
Ta có: \(\frac{a+b}{\sqrt{a\left(3a+b\right)}+\sqrt{b\left(3b+a\right)}}\)
\(=\frac{2\left(a+b\right)}{\sqrt{4a\left(3a+b\right)}+\sqrt{4b\left(3b+a\right)}}\ge\frac{2\left(a+b\right)}{\frac{1}{2}\left(4a+3a+b\right)+\frac{1}{2}\left(4b+3b+a\right)}\) (Cauchy)
\(=\frac{2\left(a+b\right)}{4\left(a+b\right)}=\frac{1}{2}\)
Dấu "=" xảy ra khi: a = b
TN
22 tháng 3 2017
Áp dụng BĐT AM-GM ta có:
\(2\sqrt{a\left(3a+b\right)}=\sqrt{4a\left(3a+b\right)}\le\frac{4a+3a+b}{2}=\frac{7a+b}{2}\)
\(2\sqrt{b\left(3b+a\right)}=\sqrt{4b\left(3b+a\right)}\le\frac{4b+3b+a}{2}=\frac{7b+a}{2}\)
Suy ra \(\sqrt{b\left(3b+a\right)}+\sqrt{a\left(3a+b\right)}\le\frac{8a+8b}{4}=2\left(a+b\right)\)
\(\Rightarrow\frac{a+b}{\sqrt{b\left(3b+a\right)}+\sqrt{a\left(3a+b\right)}}\ge\frac{a+b}{2\left(a+b\right)}=\frac{1}{2}\)
DN
1
NV
Nguyễn Việt Lâm
Giáo viên
20 tháng 6 2020
\(\frac{4\left(a+b\right)}{2\sqrt{4a\left(3a+b\right)}+2\sqrt{4b\left(3b+a\right)}}\ge\frac{4\left(a+b\right)}{4a+3a+b+4b+3b+a}=\frac{4\left(a+b\right)}{8\left(a+b\right)}=\frac{1}{2}\)
Dấu "=" xảy ra khi \(a=b\)
13 tháng 7 2020
Có: a + b = ab \(\le\frac{\left(a+b\right)^2}{4}\)
=> a + b \(\ge4\)
\(\frac{1}{a^2+2a}+\frac{1}{b^2+2b}+\sqrt{\left(1+a^2\right)\left(1+b^2\right)}\)
\(\ge\frac{4}{a^2+b^2+2\left(a+b\right)}+\sqrt{\left(1+ab\right)^2}\)
\(=\frac{4}{a^2+b^2+2ab}+\left(1+a+b\right)=\frac{4}{\left(a+b\right)^2}+\left(a+b\right)+1\)
\(=\frac{4}{\left(a+b\right)^2}+\frac{a+b}{4^2}+\frac{a+b}{4^2}+\frac{7}{8}\left(a+b\right)+1\)
\(\ge3\sqrt[3]{\frac{4}{\left(a+b\right)^2}.\frac{a+b}{4^2}.\frac{a+b}{4^2}}+\frac{7}{8}.4+1=\frac{3}{4}+\frac{7}{2}+1\)
Dấu "=" xảy ra <=> a = b = 2
5 tháng 12 2015
\(\frac{1}{a+b}+\frac{1}{b+c}+\frac{1}{c+a}+\frac{1}{2\sqrt[3]{abc}}=\frac{c^2}{c^2\left(a+b\right)}+\frac{a^2}{a^2\left(b+c\right)}+\frac{b^2}{b^2\left(c+a\right)}+\frac{\left(\sqrt[3]{abc}\right)^2}{2abc}\)
Áp dụng BĐT Bun :
\(\frac{c^2}{c^2\left(a+b\right)}+\frac{a^2}{a^2\left(b+c\right)}+\frac{b^2}{b^2\left(a+c\right)}+\frac{\left(\sqrt[3]{abc}\right)^2}{2abc}\ge\frac{\left(a+b+c+\sqrt[3]{abc}\right)^2}{c^2\left(a+b\right)+a^2\left(b+c\right)+b^2\left(a+c\right)+2abc}=...\)
Dấu ''='' xảy ra khi a = b =c
N
14 tháng 12 2020
Ta thấy: \(\Sigma_{cyc}\sqrt[3]{\frac{a^2+bc}{abc\left(b^2+c^2\right)}}=\Sigma_{cyc}\frac{a^2+bc}{\sqrt[3]{\left(a^2b+b^2c\right)\left(bc^2+ca^2\right)\left(c^2a+ab^2\right)}}\)
Ta lại có: \(\sqrt[3]{\left(a^2b+b^2c\right)\left(bc^2+ca^2\right)\left(c^2a+ab^2\right)}\le\frac{\left(a^2b+b^2c\right)+\left(bc^2+ca^2\right)+\left(c^2a+ab^2\right)}{3}=\frac{1}{3}\Sigma_{cyc}\left(ab\left(a+b\right)\right)\)
\(\Leftrightarrow\Sigma_{cyc}\sqrt[3]{\frac{a^2+bc}{abc\left(b^2+c^2\right)}}\ge\frac{\Sigma_{cyc}\left(a^2+bc\right)}{\frac{1}{3}\Sigma_{cyc}\left(ab\left(a+b\right)\right)}=\frac{a^2+b^2+c^2+ab+bc+ca}{\frac{1}{3}\Sigma_{cyc}\left(ab\left(a+b\right)\right)}\)
Nhận thấy: \(A=\left(a+b+c\right)\left(a^2+b^2+c^2+ab+bc+ca\right)=a^3+b^3+c^3+3abc+2\Sigma_{cyc}\left(ab\left(a+b\right)\right)\)
Theo Schur: \(a^3+b^3+c^3+3abc\ge\Sigma_{cyc}\left(ab\left(a+b\right)\right)\)
\(\Leftrightarrow A\ge3\Sigma_{cyc}\left(ab\left(a+b\right)\right)\)
\(\Rightarrow\Sigma_{cyc}\sqrt[3]{\frac{a^2+bc}{abc\left(b^2+c^2\right)}}\ge\frac{3\Sigma_{cyc}\left(ab\left(a+b\right)\right)}{\frac{1}{3}\left(a+b+c\right)\Sigma_{cyc}\left(ab\left(a+b\right)\right)}=\frac{9}{a+b+c}\)
\(\Leftrightarrow\left(a+b\right)\left(\frac{a+b}{2}+\frac{1}{4}\right)-\sqrt{ab}\left(\sqrt{a}+\sqrt{b}\right)\ge0\left(1\right)\)
Áp dụng BĐT Cô-si cho 2 số dương ta được:
\(VT\left(1\right)\ge\sqrt{ab}\left(a+b+\frac{1}{2}-\sqrt{a}-\sqrt{b}\right)\ge\sqrt{ab}\left[\left(\sqrt{a}-\frac{1}{2}\right)^2+\left(\sqrt{a}-\frac{1}{2}\right)^2\right]\ge0\)
\(\Rightarrow\left(1\right)\) Đúng.
\(\RightarrowĐpcm\)