sai đề: cho \(a,b\ge1\) mới chuẩn

Ta có :

\(\left(a-1\right)^2\ge0\)

\(\Rightarrow a^2+1\ge2a\)

\(\Rightarrow2a-1\le a^2\)

\(\Rightarrow\dfrac{a}{2a-1}\ge\dfrac{a}{a^2}\)

\(\Rightarrow\dfrac{a}{2a-1}\ge\dfrac{1}{a}\)

Tương tự ta có :

\(\dfrac{b}{2b-1}\ge\dfrac{1}{b}\)

Do đó : \(\dfrac{a}{2a-1}+\dfrac{b}{2b-1}\ge\dfrac{1}{a}+\dfrac{1}{b}\)

*) Chứng minh bổ đề : \(\dfrac{1}{x}+\dfrac{1}{y}\ge\dfrac{4}{x+y}\left(x,y>0\right)\)

Ta có :

\(\left(x-y\right)^2\ge0\)

\(\Rightarrow x^2+y^2\ge2xy\)

\(\Rightarrow\left(x+y\right)^2\ge4xy\)

\(\Rightarrow\dfrac{\left(x+y\right)^2}{xy\left(x+y\right)}\ge\dfrac{4xy}{xy\left(x+y\right)}\left(x,y>0\right)\)

\(\Rightarrow\dfrac{x+y}{xy}\ge\dfrac{4}{x+y}\)

\(\Rightarrow\dfrac{1}{x}+\dfrac{1}{y}\ge\dfrac{4}{x+y}\)

*) Áp dụng bổ đè trên ta có:

\(\dfrac{a}{2a-1}+\dfrac{b}{2b-1}\ge\dfrac{1}{a}+\dfrac{1}{b}\ge\dfrac{4}{a+b}\) (1)

Ta có :

\(a+b-\left(1+ab\right)\)

\(=\left(a-ab\right)+\left(b-1\right)\)

\(=a\left(1-b\right)+\left(b-1\right)\)

\(=\left(1-b\right)\left(a-1\right)\)

Vì \(a,b\ge1\Rightarrow\left\{{}\begin{matrix}a-1\ge0\\1-b\le0\end{matrix}\right.\)

\(\Rightarrow\left(1-b\right)\left(a-1\right)\le0\)

\(\Rightarrow a+b-\left(1+ab\right)\le0\)

\(\Rightarrow a+b\le1+ab\)

\(\Rightarrow\dfrac{4}{a+b}\ge\dfrac{4}{1+ab}\) (2)

Từ (1) và (2) ta được:

\(\dfrac{a}{2a-1}+\dfrac{b}{2b-1}\ge\dfrac{1}{a}+\dfrac{1}{b}\ge\dfrac{4}{a+b}\ge\dfrac{4}{1+ab}\)

\(\Rightarrow\dfrac{a}{2a-1}+\dfrac{b}{2b-1}\ge\dfrac{4}{1+ab}\)

\(\rightarrowđpcm\)

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

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

Tương tự và cộng lại:

\(VT\ge ab+bc+ca+2\left(a^2+b^2+c^2\right)=2+ab+bc+ca\)

Đặt \(\left(\sqrt{a};\sqrt{b};\sqrt{c}\right)=\left(x;y;z\right)\Rightarrow x+y+z=1\)

BĐT trở thành: \(\dfrac{xy}{\sqrt{x^2+y^2+2z^2}}+\dfrac{yz}{\sqrt{y^2+z^2+2x^2}}+\dfrac{zx}{\sqrt{x^2+z^2+2y^2}}\le\dfrac{1}{2}\)

Ta có:

\(x^2+z^2+y^2+z^2\ge\dfrac{1}{2}\left(x+z\right)^2+\dfrac{1}{2}\left(y+z\right)^2\ge\left(x+z\right)\left(y+z\right)\)

\(\Rightarrow\dfrac{xy}{\sqrt{x^2+y^2+2z^2}}\le\dfrac{xy}{\sqrt{\left(x+z\right)\left(y+z\right)}}\le\dfrac{1}{2}\left(\dfrac{xy}{x+z}+\dfrac{xy}{y+z}\right)\)

Tương tự: \(\dfrac{yz}{\sqrt{y^2+z^2+2x^2}}\le\dfrac{1}{2}\left(\dfrac{yz}{x+y}+\dfrac{yz}{x+z}\right)\)

\(\dfrac{zx}{\sqrt{z^2+x^2+2y^2}}\le\dfrac{1}{2}\left(\dfrac{zx}{x+y}+\dfrac{zx}{y+z}\right)\)

Cộng vế với vế:

\(VT\le\dfrac{1}{2}\left(\dfrac{zx+yz}{x+y}+\dfrac{xy+zx}{y+z}+\dfrac{yz+xy}{z+x}\right)=\dfrac{1}{2}\left(x+y+z\right)=\dfrac{1}{2}\) (đpcm)

Dấu "=" xảy ra khi \(x=y=z\) hay \(a=b=c\)

a) Điều phải chứng minh tương đương với:

\(a^3+b^3-a^2b-b^2a\ge0\\ \Leftrightarrow a^2\left(a-b\right)+b^2\left(b-a\right)\ge0\\ \Leftrightarrow\left(a-b\right)\left(a^2-b^2\right)\ge0\\ \Leftrightarrow\left(a-b\right)^2\left(a+b\right)\ge0\left(luon.dung\right)\)

Dấu = xảy ra khi a=b

b) Áp dụng bất đẳng thức ở phần a ta có:

\(\dfrac{1}{a^3+b^3+1}\le\dfrac{1}{a^2b+b^2a+abc}=\dfrac{1}{ab\left(a+b+c\right)}\\ =\dfrac{abc}{ab\left(a+b+c\right)}=\dfrac{c}{a+b+c}\left(do.abc=1\right)\)

Tương tự : \(\dfrac{1}{b^3+c^3+1}\le\dfrac{a}{a+b+c};\dfrac{1}{c^3+a^3+1}\le\dfrac{b}{a+b+c}\)

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

Dấu = xảy ra  <=> a=b=c=1

Câu hỏi t/tự

Lời giải:

Áp dụng BĐT Cauchy-Schwarz:

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

\(\Leftrightarrow \text{VT}\geq \frac{\left ( \frac{a^2+b^2+c^2}{abc} \right )^2}{\frac{1}{a}+\frac{1}{b}+\frac{1}{c}}\)

Theo hệ quả của BĐT AM-GM thì:

\(a^2+b^2+c^2\geq ab+bc+ac\)

\(\Rightarrow \text{VT}\geq \frac{\left ( \frac{ab+bc+ac}{abc} \right )^2}{\frac{1}{a}+\frac{1}{b}+\frac{1}{c}}=\frac{\left ( \frac{1}{a}+\frac{1}{b}+\frac{1}{c} \right )^2}{\frac{1}{a}+\frac{1}{b}+\frac{1}{c}}=\frac{1}{a}+\frac{1}{b}+\frac{1}{c}\)

Ta có đpcm.

Dấu bằng xảy ra khi \(a=b=c\)

\(\dfrac{1}{a+3b}+\dfrac{1}{a+b+2c}\ge\dfrac{4}{2a+4b+2c}=\dfrac{2}{a+2b+c}\)

Tương tự: \(\dfrac{1}{b+3c}+\dfrac{1}{b+c+2a}\ge\dfrac{2}{a+b+2c}\)

\(\dfrac{1}{c+3a}+\dfrac{1}{a+c+2b}\ge\dfrac{2}{2a+b+c}\)

Cộng vế với vế và rút gọn:

\(\dfrac{1}{a+3b}+\dfrac{1}{b+3c}+\dfrac{1}{c+3a}\ge\dfrac{1}{a+2b+c}+\dfrac{1}{b+2c+a}+\dfrac{1}{c+2a+b}\)

Dấu "=" xảy ra khi \(a=b=c\)

Áp dụngk BĐt cô-si, ta có 

\(\frac{a^2}{b^2c}+\frac{b^2}{c^2a}+\frac{1}{a}\ge3.\frac{1}{c}\)

Tương tự , rồi cộng vào, ta có 

\(2A+\frac{1}{a}+\frac{1}{b}+\frac{1}{c}\ge3\left(\frac{1}{a}+\frac{1}{b}+\frac{1}{c}\right)\Rightarrow A\ge\frac{1}{a}+\frac{1}{b}+\frac{1}{c}\left(ĐPCM\right)\)

^_^