-Áp dụng BĐT Caushy Schwarz ta có:

\(\dfrac{1}{a}+\dfrac{1}{b}+\dfrac{1}{c}\ge\dfrac{\left(1+1+1\right)^2}{a+b+c}=\dfrac{9}{1}=9\)

-Dấu "=" xảy ra khi \(a=b=c=\dfrac{1}{3}\)

\(\left(a+b+c\right)\left(\dfrac{1}{a}+\dfrac{1}{b}+\dfrac{1}{c}\right)\)

\(=1+\dfrac{a}{b}+\dfrac{a}{c}+\dfrac{b}{a}+1+\dfrac{b}{c}+\dfrac{c}{a}+\dfrac{c}{b}+1\)

\(=\left(\dfrac{a}{b}+\dfrac{b}{a}\right)+\left(\dfrac{a}{c}+\dfrac{c}{a}\right)+\left(\dfrac{b}{c}+\dfrac{c}{b}\right)+3\)

Áp dụng BĐT Cô - si cho 2 số không âm:

\(\dfrac{a}{b}+\dfrac{b}{a}\ge2\sqrt{\dfrac{a}{b}.\dfrac{b}{a}}=2\)

\(\dfrac{a}{c}+\dfrac{c}{a}\ge2\sqrt{\dfrac{a}{c}.\dfrac{c}{a}}=2\)

\(\dfrac{b}{c}+\dfrac{c}{b}\ge2\sqrt{\dfrac{b}{c}.\dfrac{c}{b}}=2\)

Suy ra:

\(\dfrac{a}{b}+\dfrac{b}{a}+\dfrac{a}{c}+\dfrac{c}{a}+\dfrac{b}{c}+\dfrac{c}{b}+3\ge2+2+2+3=9\)

Dấu "=" xảy ra khi: a = b = c

Áp dụng BĐT Cauchy dạng Engel , ta có :

\(\dfrac{1}{a}+\dfrac{1}{b}+\dfrac{1}{c}\) ≥ \(\dfrac{9}{a+b+c}\)

⇔ \(\left(a+b+c\right)\left(\dfrac{1}{a}+\dfrac{1}{b}+\dfrac{1}{c}\right)\text{≥}\left(a+b+c\right).\dfrac{9}{a+b+c}\)

⇔ \(\left(a+b+c\right)\left(\dfrac{1}{a}+\dfrac{1}{b}+\dfrac{1}{c}\right)\text{≥}9\)

\("="\text{⇔}a=b=c\)

Áp dụng bất đẳng thức Cô - si cho 3 số không âm ta có :

\(a+b+c\ge3\sqrt[3]{abc}=3\)

\(\dfrac{1}{a}+\dfrac{1}{b}+\dfrac{1}{c}\ge3\sqrt[3]{\dfrac{1}{abc}}=3\)

Nhân vế theo vế ta có :

\(\left(a+b+c\right)\left(\dfrac{1}{a}+\dfrac{1}{b}+\dfrac{1}{c}\right)\ge9\)

Dấu " = " xảy ra khi a = b = c .

Áp dụng BĐT Cauchy-Schwarz dạng Engel ta có:

\(VT=\dfrac{1}{a^2+2bc}+\dfrac{1}{b^2+2ac}+\dfrac{1}{c^2+2ab}\)

\(\ge\dfrac{\left(1+1+1\right)^2}{a^2+2bc+b^2+2ac+c^2+2ab}\)

\(=\dfrac{3^2}{\left(a+b+c\right)^2}=\dfrac{9}{\left(a+b+c\right)^2}=9\left(a+b+c\le1\right)\)

Đẳng thức xảy ra khi \(a=b=c=\dfrac{1}{3}\)

\(\dfrac{1}{a}+\dfrac{1}{b}+\dfrac{1}{c}=\dfrac{a+b+c}{a}+\dfrac{a+b+c}{b}+\dfrac{a+b+c}{c}=3+\left(\dfrac{b}{a}+\dfrac{a}{b}\right)+\left(\dfrac{c}{a}+\dfrac{a}{c}\right)+\left(\dfrac{b}{c}+\dfrac{c}{b}\right)\)Ta có: \(\dfrac{a}{b}+\dfrac{b}{a}\ge2;\dfrac{c}{a}+\dfrac{a}{c}\ge2;\dfrac{b}{c}+\dfrac{c}{b}\ge2\)

\(\Leftrightarrow\)\(\dfrac{1}{a}+\dfrac{1}{b}+\dfrac{1}{c}\ge3+2+2+2=9\)

Gọi \(A=\dfrac{1}{a}+\dfrac{1}{b}+\dfrac{1}{c}\) Ta có:

\(A=\dfrac{1}{a}+\dfrac{1}{b}+\dfrac{1}{c}=\dfrac{a+b+c}{a}+\dfrac{a+b+c}{b}+\dfrac{a+b+c}{c}\)\(=\left(\dfrac{a}{a}+\dfrac{b}{a}+\dfrac{c}{a}\right)+\left(\dfrac{a}{b}+\dfrac{b}{b}+\dfrac{c}{b}\right)+\left(\dfrac{a}{c}+\dfrac{b}{c}+\dfrac{c}{c}\right)\)

\(=\left(1+\dfrac{b}{a}+\dfrac{c}{a}\right)+\left(\dfrac{a}{b}+1+\dfrac{c}{b}\right)+\left(\dfrac{a}{c}+\dfrac{b}{c}+1\right)\)

\(=3+\left(\dfrac{a}{b}+\dfrac{b}{a}\right)+\left(\dfrac{b}{c}+\dfrac{c}{b}\right)+\left(\dfrac{c}{a}+\dfrac{a}{c}\right)\)

\(=3+\left(\dfrac{a^2}{ab}+\dfrac{b^2}{ab}\right)+\left(\dfrac{b^2}{bc}+\dfrac{c^2}{bc}\right)+\left(\dfrac{c^2}{ac}+\dfrac{a^2}{ac}\right)\)

\(=3+\dfrac{a^2+b^2}{ab}+\dfrac{b^2+c^2}{bc}+\dfrac{c^2+a^2}{ac}\)

\(=3+\dfrac{a^2-2ab+b^2+2ab}{ab}+\dfrac{b^2-2bc+c^2+2bc}{bc}+\dfrac{c^2-2ac+a^2+2ac}{ac}\)

\(=3+\dfrac{\left(a-b\right)^2+2ab}{ab}+\dfrac{\left(b-c\right)^2+2bc}{bc}+\dfrac{\left(c-a\right)^2+2ac}{ac}\)

\(=3+\dfrac{\left(a-b\right)^2}{ab}+2+\dfrac{\left(b-c\right)^2}{bc}+2+\dfrac{\left(c-a\right)^2}{ac}+2\)

\(=9+\dfrac{\left(a-b\right)^2}{ab}+\dfrac{\left(b-c\right)^2}{bc}+\dfrac{\left(c-a\right)^2}{ac}\)

Ta thấy: \(\dfrac{\left(a-b\right)^2}{ab}\ge0\) với \(\forall\) a, b

\(\dfrac{\left(b-c\right)^2}{bc}\ge0\) với \(\forall\) b, c

\(\dfrac{\left(c-a\right)^2}{ac}\ge\) 0 với \(\forall\) a, c

=> \(A\ge9\).

Vậy...

Chúc bạn học tốt!

Ta có \(a+b+c=1\)

Suy ra:

\(1+\dfrac{b}{a}+\dfrac{c}{a}=\dfrac{1}{a}\\ \dfrac{a}{b}+1+\dfrac{c}{b}=\dfrac{1}{b}\\ \dfrac{a}{c}+\dfrac{b}{c}+1=\dfrac{1}{c}\)

Cộng vế với vế các phương trình trên ta được:

\(\dfrac{1}{a}+\dfrac{1}{b}+\dfrac{1}{c}=3+\dfrac{b}{a}+\dfrac{a}{b}+\dfrac{c}{a}+\dfrac{a}{c}+\dfrac{c}{b}+\dfrac{b}{c}\)

Áp dụng bất đẳng thức Cô-si cho 3 số a, b, c dương:

\(\dfrac{b}{a}+\dfrac{a}{b}\ge2\sqrt{\dfrac{b}{a}\cdot\dfrac{a}{b}}=2\\ \dfrac{c}{a}+\dfrac{a}{c}\ge2\sqrt{\dfrac{c}{a}\cdot\dfrac{a}{c}}=2\\ \dfrac{c}{b}+\dfrac{b}{c}\ge2\sqrt{\dfrac{c}{b}\cdot\dfrac{b}{c}}=2\)

Từ đó ta suy ra:

\(3+\dfrac{b}{a}+\dfrac{a}{b}+\dfrac{c}{a}+\dfrac{a}{c}+\dfrac{c}{b}+\dfrac{b}{c}\ge3+2\sqrt{\dfrac{b}{a}\cdot\dfrac{a}{b}}+2\sqrt{\dfrac{c}{a}\cdot\dfrac{a}{c}}+2\sqrt{\dfrac{c}{b}\cdot\dfrac{b}{c}}=3+2+2+2=9\)

Hay \(\dfrac{1}{a}+\dfrac{1}{b}+\dfrac{1}{c}\ge9\Rightarrow\)đpcm

Đầu tiên ta cm:\(\dfrac{1}{a}+\dfrac{1}{b}+\dfrac{1}{c}\ge\dfrac{9}{a+b+c}\)(tự cm)

Áp dụng:\(\Rightarrow\dfrac{1}{a^2+2bc}+\dfrac{1}{b^2+2ac}+\dfrac{1}{c^2+2ab}\ge\dfrac{9}{a^2+b^2+c^2+2ab+2bc+2ca}\)

Lại có:\(a^2+b^2+c^2+2ab+2bc+2ca=\left(a+b+c\right)^2\le1\)

\(\Rightarrow\dfrac{9}{a^2+b^2+c^2+2ab+2bc+2ca}\ge\dfrac{9}{1}=9\)

\(\Rightarrowđpcm\)

do \(a,b,c\ge1\)\(=>\left\{{}\begin{matrix}b+c\ge2\\c+a\ge2\\a+b\ge2\end{matrix}\right.\)

\(=>\left\{{}\begin{matrix}a\left(b+c\right)\ge2a\\b\left(c+a\right)\ge2b\\c\left(a+b\right)\ge2c\end{matrix}\right.\)

\(=>\) biểu thức đề bài cho\(\ge2\left(a+b+c+\dfrac{1}{a^2+1}+\dfrac{1}{b^2+1}+\dfrac{1}{c^2+1}\right)\)

\(2\left(1+1+1+\dfrac{1}{2}+\dfrac{1}{2}+\dfrac{1}{2}\right)=9\)

dấu= xảy ra<=>a=b=c=1

\(a;b;c\ge1\Rightarrow\left\{{}\begin{matrix}ab;bc;ca\ge1\\ab+bc+ca\ge3\end{matrix}\right.\)

Ta có:

\(\dfrac{1}{a^2+1}+\dfrac{1}{b^2+1}=\dfrac{a^2+b^2+2}{a^2b^2+a^2+b^2+1}=1-\dfrac{a^2b^2-1}{a^2b^2+a^2+b^2+1}\ge1-\dfrac{a^2b^2-1}{a^2b^2+2ab+1}=\dfrac{2}{ab+1}\)

Tương tự: \(\dfrac{1}{a^2+1}+\dfrac{1}{c^2+1}\ge\dfrac{2}{ac+1}\) ; \(\dfrac{1}{b^2+1}+\dfrac{1}{c^2+1}\ge\dfrac{2}{bc+1}\)

Cộng vế: \(\dfrac{1}{a^2+1}+\dfrac{1}{b^2+1}+\dfrac{1}{c^2+1}\ge\dfrac{1}{ab+1}+\dfrac{1}{bc+1}+\dfrac{1}{ca+1}\)

Do đó: \(VT\ge2\left(ab+bc+ca+\dfrac{1}{ab+1}+\dfrac{1}{bc+1}+\dfrac{1}{ca+1}\right)\)

\(VT\ge2\left(ab+bc+ca+\dfrac{9}{ab+bc+ca+3}\right)\)

Đặt \(ab+bc+ca=x\ge3\Rightarrow VT\ge2\left(x+\dfrac{9}{x+3}\right)\)

\(VT\ge2\left(\dfrac{x+3}{4}+\dfrac{9}{x+3}+\dfrac{3x}{4}-\dfrac{3}{4}\right)\ge2\left(2\sqrt{\dfrac{9\left(x+3\right)}{4\left(x+3\right)}}+\dfrac{3}{4}.3-\dfrac{3}{4}\right)=9\)

\(\left(a+b+c\right)\left(\dfrac{1}{a}+\dfrac{1}{b}+\dfrac{1}{c}\right)\)

=\(1+\dfrac{a}{b}+\dfrac{a}{c}+\dfrac{b}{a}+1+\dfrac{b}{c}+\dfrac{c}{a}+\dfrac{c}{b}+1\)

= \(3+\left(\dfrac{a}{b}+\dfrac{b}{a}\right)+\left(\dfrac{a}{c}+\dfrac{c}{a}\right)+\left(\dfrac{b}{c}+\dfrac{c}{b}\right)\)

\(\ge3+2\sqrt{\dfrac{a}{b}.\dfrac{b}{a}}+2\sqrt{\dfrac{a}{c}.\dfrac{c}{a}}+2\sqrt{\dfrac{b}{c}.\dfrac{c}{b}}\)

\(\ge3+2+2+2=9\left(đpcm\right)\)

vì a,b,c là các số dương nên ta có:

\(a+b+c\ge3\sqrt[3]{abc}\)

\(\dfrac{1}{a}+\dfrac{1}{b}+\dfrac{1}{c}\ge3\sqrt[3]{\dfrac{1}{abc}}\)

nhân hai vế vs nhau, ta có

\(\left(a+b+c\right)\left(\dfrac{1}{a}+\dfrac{1}{b}+\dfrac{1}{c}\right)\ge9\)