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1) c/m \(a+b+c\ge\sqrt{ab}+\sqrt{bc}+\sqrt{ac}\)
áp dụng BĐT cô shi cho 2 số thực dương ta có:
\(a+b\ge2\sqrt{ab}\);\(b+c\ge2\sqrt{bc}\);\(a+c\ge2\sqrt{ac}\)
cộng vế vs vế:\(2\left(a+b+c\right)\ge2\left(\sqrt{ab}+\sqrt{bc}+\sqrt{ca}\right)\)
↔\(a+b+c\ge\sqrt{ab}+\sqrt{bc}+\sqrt{ca}\)
dấu = xảy ra khi a=b=c
vậy...
b)ta có:
\(\frac{1}{\sqrt{1}}>\frac{1}{\sqrt{2}}>\frac{1}{\sqrt{3}}>...>\frac{1}{\sqrt{25}}\)→\(A>\frac{1}{\sqrt{25}}+\frac{1}{\sqrt{25}}+...+\frac{1}{\sqrt{25}}\)(25 số hạng)
\(A>\frac{25}{\sqrt{25}}=\sqrt{25}=5\)
vậy.....
1) \(\Sigma\frac{a}{b^3+ab}=\Sigma\left(\frac{1}{b}-\frac{b}{a+b^2}\right)\ge\Sigma\frac{1}{a}-\Sigma\frac{1}{2\sqrt{a}}=\Sigma\left(\frac{1}{a}-\frac{2}{\sqrt{a}}+1\right)+\Sigma\frac{3}{2\sqrt{a}}-3\)
\(\ge\Sigma\left(\frac{1}{\sqrt{a}}-1\right)^2+\frac{27}{2\left(\sqrt{a}+\sqrt{b}+\sqrt{c}\right)}-3\ge\frac{27}{2\sqrt{3\left(a+b+c\right)}}-3=\frac{3}{2}\)
1,
\(\frac{a}{1+\frac{b}{a}}+\frac{b}{1+\frac{c}{b}}+\frac{c}{1+\frac{a}{c}}=\frac{a^2}{a+b}+\frac{b^2}{b+c}+\frac{c^2}{c+a}\ge\frac{\left(a+b+c\right)^2}{2\left(a+b+c\right)}=\frac{a+b+c}{2}\ge\frac{\sqrt{ab}+\sqrt{bc}+\sqrt{ca}}{2}=\frac{2}{2}=1\left(Q.E.D\right)\)
\(A=\frac{\frac{1}{2}a^2\left(\sqrt[3]{b}+\sqrt[3]{c}+1\right)\left[\left(\sqrt[3]{b}-\sqrt[3]{c}\right)^2+\left(\sqrt[3]{b}-1\right)^2+\left(\sqrt[3]{c}-1\right)^2\right]}{2\left(a+2\right)\left(a+\sqrt[3]{bc}\right)}\ge0\)
\(\Sigma_{cyc}\frac{a^2}{a+\sqrt[3]{bc}}=\Sigma_{cyc}A+\Sigma_{cyc}\frac{2\left(a-1\right)^2}{3\left(a+2\right)}+\frac{5}{6}\left(a+b+c\right)-1\ge\frac{5}{6}\left(a+b+c\right)-1=\frac{3}{2}\)
Áp dụng bất đẳng thức cộng mẫu số
\(\Rightarrow\frac{a^2}{a+\sqrt[3]{bc}}+\frac{b^2}{b+\sqrt[3]{ca}}+\frac{c^2}{c+\sqrt[3]{ab}}\)\(\ge\frac{\left(a+b+c\right)^2}{a+b+c+\sqrt[3]{bc}+\sqrt[3]{ca}+\sqrt[3]{ab}}\)
\(\Rightarrow\frac{a^2}{a+\sqrt[3]{bc}}+\frac{b^2}{b+\sqrt[3]{ca}}+\frac{c^2}{c+\sqrt[3]{ab}}\)\(\ge\frac{9}{3+\sqrt[3]{bc}+\sqrt[3]{ca}+\sqrt[3]{ab}}\)
Chứng minh rằng : \(\frac{9}{3+\sqrt[3]{bc}+\sqrt[3]{ca}+\sqrt[3]{ab}}\ge\frac{3}{2}\)
\(\Leftrightarrow18\ge3\left(3+\sqrt[3]{bc}+\sqrt[3]{ca}+\sqrt[3]{ab}\right)\)
\(\Leftrightarrow18\ge9+3\sqrt[3]{bc}+3\sqrt[3]{ca}+3\sqrt[3]{ab}\)
\(\Leftrightarrow9\ge3\sqrt[3]{ab}+3\sqrt[3]{bc}+3\sqrt[3]{ca}\)
Áp dụng bất đẳng thức Cauchy cho 3 bộ số thực không âm
\(\Rightarrow\hept{\begin{cases}a+b+1\ge3\sqrt[3]{ab}\\b+c+1\ge3\sqrt[3]{bc}\\c+a+1\ge3\sqrt[3]{ca}\end{cases}}\)
\(\Rightarrow2\left(a+b+c\right)+3\ge3\sqrt[3]{ab}+3\sqrt[3]{bc}+3\sqrt[3]{ca}\)
\(\Rightarrow9\ge3\sqrt[3]{ab}+3\sqrt[3]{bc}+3\sqrt[3]{ca}\left(đpcm\right)\)
Vì \(\frac{9}{3+\sqrt[3]{bc}+\sqrt[3]{ca}+\sqrt[3]{ab}}\ge\frac{3}{2}\)
Mà \(\frac{a^2}{a+\sqrt[3]{bc}}+\frac{b^2}{b+\sqrt[3]{ca}}+\frac{c^2}{c+\sqrt[3]{ab}}\ge\frac{9}{3+\sqrt[3]{bc}+\sqrt[3]{ca}+\sqrt[3]{ab}}\)
\(\Rightarrow\frac{a^2}{a+\sqrt[3]{bc}}+\frac{b^2}{b+\sqrt[3]{ca}}+\frac{c^2}{c+\sqrt[3]{ab}}\ge\frac{3}{2}\left(đpcm\right)\)
Chúc bạn học tốt !!!
Với \(a,b,c\ge0\). Khi đó ta có
\(\frac{a}{b+c}+\frac{b}{c+a}+\frac{c}{a+b}\ge\frac{a^2+b^2+c^2}{ab+bc+ca}\)
Chứng minh: \(\left(ab+bc+ca\right)\left(\frac{a}{b+c}+\frac{b}{c+a}+\frac{c}{a+b}\right)=a^2+b^2+c^2+abc\left(\frac{1}{b+c}+\frac{1}{c+a}+\frac{1}{a+b}\right)\ge a^2+b^2+c^2\)\(\Rightarrow\frac{a}{b+c}+\frac{b}{c+a}+\frac{c}{a+b}\ge\frac{a^2+b^2+c^2}{ab+bc+ac}\)
Với \(a,b,c\ge0\) ta có
\(\sqrt{\frac{ab}{\left(a+c\right)\left(b+c\right)}}+\sqrt{\frac{bc}{\left(b+a\right)\left(c+a\right)}}+\sqrt{\frac{ca}{\left(c+b\right)\left(c+a\right)}}\ge1\)
Áp dụng bất đẳng thức AM-GM ta có:
\(\Sigma\sqrt{\frac{ab}{\left(a+c\right)\left(b+c\right)}}=\Sigma\sqrt{\frac{ab\left(2ab+2bc+2ac\right)^2}{4\left(a+c\right)\left(b+c\right)\left(ab+bc+ca\right)^2}}\)
\(\ge\Sigma\sqrt{\frac{ab\left[a\left(b+c\right)+b\left(a+c\right)\right]^2}{4\left(a+c\right)\left(b+c\right)\left(ab+bc+ac\right)^2}}\)
\(\ge\Sigma\sqrt{\frac{ab.4a\left(b+c\right)b\left(a+c\right)}{4\left(a+c\right)\left(b+c\right)\left(ab+bc+ca\right)^2}}=\Sigma\frac{ab}{ab+bc+ca}\)
Từ đó ta có \(\Sigma\sqrt{\frac{ab}{\left(a+c\right)\left(b+c\right)}}\ge\frac{ab+bc+ca}{ab+bc+ca}=1\)
chứng minh bài toán:
Đặt \(\sqrt{\frac{a^2+b^2+c^2}{ab+bc+ac}}=t\ge1\)
Ta có: \(\left(\Sigma\sqrt{\frac{a}{b+c}}\right)^2=\Sigma\frac{a}{b+c}+2\Sigma\sqrt{\frac{ab}{\left(a+c\right)\left(b+c\right)}}\ge\frac{a^2+b^2+c^2}{ab+bc+ac}+2=t^2+2\)
Từ đây ta chứng minh \(\sqrt{t^2+2}+\frac{3\sqrt{3}}{t}\ge\frac{7\sqrt{2}}{2}\)
Áp dụng bất đẳng thức bunhiacopxki ta có:
\(\sqrt{t^2+2}+\frac{3\sqrt{3}}{t}=\frac{\sqrt{\left(t^2+2\right)\left(6+2\right)}}{2\sqrt{2}}+\frac{3\sqrt{3}}{t}\ge\frac{t\sqrt{6}+2}{2\sqrt{2}}+\frac{3\sqrt{3}}{t}=\left(\frac{t\sqrt{3}}{2}+\frac{3\sqrt{3}}{t}\right)+\frac{\sqrt{2}}{2}\)
Áp dụng bất đẳng thức Cauchy ta đc:
\(\left(\frac{t\sqrt{3}}{2}+\frac{3\sqrt{3}}{t}\right)+\frac{\sqrt{2}}{2}\ge3\sqrt{2}+\frac{\sqrt{2}}{2}=\frac{7\sqrt{2}}{2}\)
Vậy ta có đpcm
Em thấy nó là lạ chỗ:" từ đây ta chứng minh: \(\sqrt{t^2+2}+\frac{3\sqrt{3}}{t}\ge\frac{7\sqrt{2}}{2}\)" ấy ạ, em nghĩ phải là chứng minh \(\sqrt{t^2+2}+3\sqrt{3}.t\ge\frac{7\sqrt{2}}{2}\) chứ ạ?