giả sử a\(\le\)b \(\le\)c.

khi đó \(\frac{a}{b+c}\le\frac{b}{c+a}\le\frac{c}{a+b}\)

áp dụng BĐT Trê bư sép ta có:

\(\left(a^2+b^2+c^2\right)\left(\frac{a}{b+c}+\frac{b}{c+a}+\frac{c}{a+b}\right)\le3\left(\frac{a^3}{b+c}+\frac{b^3}{c+a}+\frac{c^3}{a+b}\right)=3VT\)

lại có a² + b² + c² \(\ge\) \(\frac{\left(a+b+c\right)^2}{3}\) nên:

3VT \(\ge\frac{\left(a+b+c\right)^2}{3}\left(\frac{a}{b+c}+\frac{b}{c+a}+\frac{c}{a+b}\right)\)

hay VT \(\ge\left(\frac{a+b+c}{3}\right)^2\left(\frac{a}{b+c}+\frac{b}{c+a}+\frac{c}{a+b}\right)\). đpcm

Cho $a, b>0$.Chứng minh rằng $\frac{1}{{a^3 }} + \frac{{a^3 }}{{b^3 }} + b^3 \ge \frac{1}{a} + \frac{a}{b} + b$ - K2PI – TOÁN THPT | Chia sẻ Tài liệu, đề thi, hỗ trợ giải toán

Câu a : \(\frac{a}{b+c}+\frac{b}{c+a}+\frac{c}{a+b}\ge\frac{3}{2}\)

\(\Leftrightarrow\left(\frac{a}{b+c}+1\right)+\left(\frac{b}{c+a}+1\right)+\left(\frac{c}{a+b}+1\right)\ge\frac{9}{2}\)

\(\Leftrightarrow\left(a+b+c\right)\left(\frac{1}{a+b}+\frac{1}{b+c}+\frac{1}{c+a}\right)\ge\frac{9}{2}\)

\(VT=\left(a+b+c\right)\left(\frac{1}{a+b}+\frac{1}{b+c}+\frac{1}{c+a}\right)\ge\frac{\left(a+b+c\right).9}{2\left(a+b+c\right)}=\frac{9}{2}\) (đpcm)

Dấu "\("="\) xảy ra khi \(a=b=c\)

Câu b : \(VT=\frac{a^2}{b+c}+\frac{b^2}{c+a}+\frac{c^2}{a+b}\ge\frac{\left(a+b+c\right)^2}{2\left(a+b+c\right)}=\frac{a+b+c}{2}\left(đpcm\right)\)

Dấu = xảy ra khi a=b=c

Lời giải:

\(\text{VT}=a-\frac{ab^2}{a^2+b^2}+b-\frac{bc^2}{b^2+c^2}+c-\frac{ca^2}{a^2+c^2}=(a+b+c)-\left(\frac{ab^2}{a^2+b^2}+\frac{bc^2}{b^2+c^2}+\frac{ca^2}{c^2+a^2}\right)(1)\)

Áp dụng BĐT AM-GM:

\(\frac{ab^2}{a^2+b^2}+\frac{bc^2}{b^2+c^2}+\frac{ca^2}{c^2+a^2}\leq \frac{ab^2}{2ab}+\frac{bc^2}{2bc}+\frac{ca^2}{2ac}=\frac{a+b+c}{2}(2)\)

Từ $(1);(2)\Rightarrow \text{VT}\geq a+b+c-\frac{a+b+c}{2}=\frac{a+b+c}{2}$

Ta có đpcm.

Dấu "=" xảy ra khi $a=b=c$

sửa lại

\(A=\frac{a}{1+b^2}+\frac{b}{1+c^2}+\frac{c}{1+a^2}\)

\(=a-\frac{ab^2}{1+b^2}+b-\frac{bc^2}{1+c^2}+c-\frac{ca^2}{1+a^2}\)

áp dụng bđt cauchy ta có:

\(b^2+1\ge2b;c^2+1\ge2c;a^2+1\ge2a\)

\(\Rightarrow a-\frac{ab^2}{1+b^2}+b-\frac{bc^2}{1+c^2}+c-\frac{ca^2}{1+a^2}\ge a-\frac{ab^2}{2b}+b-\frac{bc^2}{2b}+c-\frac{ca^2}{2a}\)

\(=a+b+c-\frac{ab+bc+ca}{2}\)

áp dụng cauchy ta có:

\(\left(a+b+c\right)^2\ge3\left(ab+bc+ca\right)\Rightarrow ab+bc+ca\le\frac{\left(a+b+c\right)^2}{3}=3\)

\(\Rightarrow a+b+c-\frac{ab+bc+ca}{2}\ge3-\frac{3}{2}=\frac{3}{2}\)

\(\Rightarrow\frac{a}{1+b^2}+\frac{b}{1+c^2}+\frac{c}{1+a^2}\ge\frac{3}{2}\left(Q.E.D\right)\)

dấu bằng xảy ra khi a=b=c=1

đặt \(A=\frac{a}{1+b^2}+\frac{b}{1+c^2}+\frac{c}{1+a^2}=a-\frac{ab^2}{1+b^2}+b-\frac{bc^2}{1+c^2}+c-\frac{ca^2}{1+a^2}\)

\(=\left(a+b+c\right)-\left(\frac{ab^2}{b^2+1}+\frac{bc^2}{c^2+1}+\frac{ca^2}{a^2+1}\right)\le3-\left(\frac{ab^2}{2b}+\frac{bc^2}{2c}+\frac{ca^2}{2a}\right)=3-\left(\frac{ab+bc+ca}{2}\right)\ge3-\frac{\left(a+b+c\right)^2}{6}=\frac{3}{2}\left(Q.E.D\right)\)

#)Giải : 

Áp dụng BĐT Cauchy : 

\(\frac{ab}{c}+\frac{bc}{a}\ge2.\sqrt{\frac{ab}{c}.\frac{bc}{a}}=2b\left(1\right)\)

Chứng minh tương tự, ta được :

\(\frac{bc}{a}+\frac{ca}{b}\ge2c\left(2\right)\)

\(\frac{ab}{c}+\frac{ca}{b}\ge2a\left(3\right)\)

Từ \(\left(1\right)\left(2\right)\left(3\right)\)\(\Rightarrow2\left(\frac{ab}{c}+\frac{bc}{a}+\frac{ca}{b}\right)\ge2\left(a+b+c\right)\)

\(\Rightarrow\frac{ab}{c}+\frac{bc}{a}+\frac{ca}{b}\ge a+b+c\left(đpcm\right)\)

Áp dụng BĐT AM-GM ta có:

\(\hept{\begin{cases}\frac{bc}{a}+\frac{ac}{b}\ge2.\sqrt{\frac{bc}{a}.\frac{ac}{b}}=2.c\\\frac{bc}{a}+\frac{ab}{c}\ge2.\sqrt{\frac{bc}{a}.\frac{ab}{c}}=2b\\\frac{ac}{b}+\frac{ab}{c}\ge2.\sqrt{\frac{ac}{b}.\frac{ab}{c}}=2a\end{cases}}\Leftrightarrow2.\left(\frac{bc}{a}+\frac{ac}{b}+\frac{ab}{c}\right)\ge2\left(a+b+c\right)\)

\(\Leftrightarrow\frac{bc}{a}+\frac{ac}{b}+\frac{ab}{c}\ge a+b+c\)

Dấu " = " xảy ra \(\Leftrightarrow a=b=c\)( tự giải rõ ra nhé )

BĐT AM-GM: 

\(a+a_1+a_2+...+a_n\ge n\sqrt[n]{a.a_1.a_2.....a_n}\)

Dấu " = " xảy ra \(\Leftrightarrow a=a_1=a_2=...=a_n\)

\(\frac{bc}{a}+\frac{ac}{b}+\frac{ab}{c}\ge a+b+c\)

\(\Leftrightarrow\frac{abc}{a^2}+\frac{abc}{b^2}+\frac{abc}{c^2}\ge a+b+c\)

\(\Leftrightarrow abc.\left(\frac{1}{a^2}+\frac{1}{b^2}+\frac{1}{c^2}\right)\ge a+b+c\)

Giải tiếp nhé

Lời giải:

\(\text{BĐT}\Leftrightarrow \frac{\frac{a^3}{b}+\frac{b^3}{c}+\frac{c^3}{a}}{abc}\geq\frac{ab+bc+ac}{abc}\)

\(\Leftrightarrow \frac{a^3}{b}+\frac{b^3}{c}+\frac{c^3}{a}\geq ab+bc+ac\) \((\star)\)

Điều này hiển nhiên đúng vì theo Cauchy-SChwarz kết hợp AM-GM:

\(\text{VT}_{\star}=\frac{a^4}{ab}+\frac{b^4}{bc}+\frac{c^4}{ac}\geq \frac{(a^2+b^2+c^2)^2}{ab+bc+ac}\geq ab+bc+ac\)

Do đó ta có đpcm

Dấu bằng xảy ra khi $a=b=c$

Áp dụng bất đẳng thức Cauchy-Schwarz ta có :

\(VT\ge\frac{\left(a+b+c\right)^2}{4\left(a+b+c\right)}=\frac{a+b+c}{4}\)

Dấu đẳng thức xảy ra \(\Leftrightarrow a=b=c\)