De dung la:

\(\Sigma_{cyc}\frac{1}{1+a^2+b^2}\le\frac{9}{5}\)

\(\Leftrightarrow\Sigma_{cyc}\frac{a^2+b^2}{1+a^2+b^2}\ge\frac{6}{5}\)

\(VT\ge\frac{\left(\Sigma_{cyc}\sqrt{a^2+b^2}\right)^2}{2\Sigma_{cyc}a^2+3}\left(M\right)\)

Consider:

\(VT_M\ge\frac{6}{5}\)

\(5\Sigma_{cyc}\sqrt{\left(a^2+b^2\right)\left(b^2+c^2\right)}\ge\Sigma_{cyc}a^2+9\)

Consider:

\(5\Sigma_{cyc}\sqrt{\left(a^2+b^2\right)\left(b^2+c^2\right)}\ge5\Sigma_{cyc}a^2+5\Sigma_{cyc}ab=5\Sigma_{cyc}a^2+5\)

Gio can cung minh:

\(5\Sigma_{cyc}a^2+5\ge\Sigma_{cyc}a^2+9\)

\(\Leftrightarrow\Sigma_{cyc}a^2\ge1\)

Ta lai co:

\(\Sigma_{cyc}a^2\ge\Sigma_{cyc}ab=1\)

Dau '=' xay ra khi \(a=b=c=\frac{1}{\sqrt{3}}\)

CM cái sau: 

Ta có: \(a+\frac{1}{a}=\frac{a}{1}+\frac{1}{a}\ge2\sqrt{\frac{a}{1}.\frac{1}{a}}=2.1=2\) (bất đẳng thức Cauchy)

Chứng minh: 

\(\left(a-b\right)^2\ge0\left(\forall a,b\right)\)

\(\Leftrightarrow a^2-2ab+b^2\ge0\)

\(\Leftrightarrow a^2+2ab+b^2\ge4ab\)

\(\Leftrightarrow\left(a+b\right)^2\ge4ab\)

\(\Leftrightarrow a+b\ge2\sqrt{ab}\)

(áp dụng vào cái trên)

Dấu "=" xảy ra khi:

\(a=\frac{1}{a}\Leftrightarrow a^2=1\Rightarrow a=1\left(a>0\right)\)

Ta có: \(\frac{a}{1+b^2}=\frac{a\left(1+b^2\right)-ab^2}{1+b^2}=a-\frac{ab}{1+b^2}\)

\(1+b^2\ge2b\) \(\Rightarrow\frac{ab^2}{1+b^2}\le\frac{ab^2}{2b}=\frac{ab}{2}\)\(\Rightarrow-\frac{ab^2}{1+b^2}\ge-\frac{ab}{2}\)

Do đó: \(\frac{a}{1+b^2}=a-\frac{ab^2}{1+b^2}\ge a-\frac{ab}{2}\)

Tương tự: \(\frac{b}{1+c^2}\ge b-\frac{bc}{2}\);  \(\frac{c}{1+a^2}\ge c-\frac{ca}{2}\)

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

Mặt khác ta có: \(3\ge\frac{1}{a}+\frac{1}{b}+\frac{1}{c}\ge\frac{9}{a+b+c}\Rightarrow\frac{3}{a+b+c}\le1\)

\(\Rightarrow a+b+c\ge3\)

Do đó; \(\frac{a}{1+b^2}+\frac{b}{1+c^2}+\frac{c}{1+a^2}+\frac{ab+bc+ca}{2}\ge a+b+c\ge3\)(đpcm)

Dấu "=" xảy ra khi và chỉ khi \(a=b=c=1\)

a,b,c< 0 mà a+b+c bé hơn hoặc bằng 1

a+b+c ít nhất phải bằng 3 chứ!

a) Áp dụng BĐT Svácxơ, ta có:

\(\dfrac{1}{a}+\dfrac{1}{b}+\dfrac{1}{c}\ge\dfrac{\left(1+1+1\right)^2}{a+b+c}=\dfrac{9}{6}=\dfrac{3}{2}\)

Dấu "=" \(\Leftrightarrow a=b=c=2\)

b) Áp dụng BĐT Svácxơ, ta có:

\(\dfrac{a^2}{c}+\dfrac{b^2}{a}+\dfrac{c^2}{b}\ge\dfrac{\left(a+b+c\right)^2}{a+b+c}=a+b+c=6\)

Dấu "=" \(\Leftrightarrow a=b=c=2\)

đặt \(\sqrt{\frac{ab}{c}}=x;\sqrt{\frac{bc}{a}}=y;\sqrt{\frac{ca}{b}}=z\Rightarrow xy+yz+zx=1\)

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

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

\(\ge\frac{\left(x+y+z\right)^2}{\left(x+y+z\right)^2+\frac{\left(x+y+z\right)^2}{3}}=\frac{3}{4}\left(Q.E.D\right)\)

\(\left(a+\frac{1}{b}\right)^2+\left(b+\frac{1}{a}\right)^2\)

\(\ge\frac{\left(a+b+\frac{1}{a}+\frac{1}{b}\right)^2}{2}\)

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

\(=\frac{25}{2}\) 

tại a=b=¹/₂

thêm ít cách

Cách 1:

Áp dụng BĐT bunhiacopxki ta được:

\(\left[\left(a+\frac{1}{b}\right)^2+\left(b+\frac{1}{a}\right)^2\right]\left(1^2+1^2\right)\ge\left[\left(a+\frac{1}{b}\right)+\left(b+\frac{1}{a}\right)\right]^2\)

\(\Leftrightarrow\left[\left(a+\frac{1}{b}\right)^2+\left(b+\frac{1}{a}\right)^2\right]2\ge\left(1+\frac{1}{a}+\frac{1}{b}\right)^2\)(1)

Ta có:\(\frac{1}{a}+\frac{1}{b}\ge\frac{2}{\sqrt{ab}}\)( tự CM nha )

ÁP dụng BĐT AM-GM ta có:

\(\sqrt{ab}\le\frac{a+b}{2}=\frac{1}{2}\)

\(\Rightarrow\frac{1}{a}+\frac{1}{b}\ge4\)(2)

Thay (2) vào (1) ta được: 

\(\left[\left(a+\frac{1}{b}\right)^2+\left(b+\frac{1}{a}\right)^2\right]2\ge25\)

\(\Rightarrow\left(a+\frac{1}{b}\right)^2+\left(b+\frac{1}{a}\right)^2\ge\frac{25}{2}\left(đpcm\right)\)

Dấu"="xảy ra \(\Leftrightarrow a=b=\frac{1}{2}\)

Cách 2: 

Đặt \(P=\left(a+\frac{1}{b}\right)^2+\left(b+\frac{1}{a}\right)^2\)

Ta có: \(\left(a+\frac{1}{b}\right)^2+\left(b+\frac{1}{a}\right)^2=a^2+\frac{2a}{b}+\frac{1}{b^2}+b^2+\frac{2b}{a}+\frac{1}{a^2}\)

\(=a^2+\frac{2a}{b}+\frac{1}{16b^2}+\frac{15}{16b^2}+b^2+\frac{2b}{a}+\frac{1}{16a^2}+\frac{15}{16a^2}\)

\(=\left(a^2+\frac{1}{16a^2}\right)+\left(b^2+\frac{1}{16b^2}\right)+\left(\frac{2a}{b}+\frac{2b}{a}\right)+\left(\frac{15}{16b^2}+\frac{15}{16a^2}\right)\)

ÁP dụng BĐT AM-GM ta có:

\(a^2+\frac{1}{16a^2}\ge2\sqrt{a^2.\frac{1}{16a^2}}\ge\frac{1}{2}\)(3)

\(b^2+\frac{1}{16b^2}\ge2\sqrt{b^2.\frac{1}{16b^2}}\ge\frac{1}{2}\)(4)

\(\frac{2a}{b}+\frac{2b}{a}\ge2\sqrt{\frac{2a}{b}.\frac{2b}{a}}\ge4\)(5)

\(\frac{15}{16a^2}+\frac{15}{16b^2}\ge2\sqrt{\frac{15.15}{16.16a^2b^2}}=\frac{15}{8ab}\)(1) 

ÁP dụng BĐT AM-GM ta có:

\(ab\le\frac{\left(a+b\right)^2}{4}=\frac{1}{4}\)(2)

Thay (2) vào (1) ta được:

\(\frac{15}{16a^2}+\frac{15}{16b^2}\ge\frac{15}{2}\)(6)

Cộng (3)+(4)+(5)+(6) ta được: 

\(P\ge\frac{1}{2}+\frac{1}{2}+\frac{15}{2}+4=\frac{25}{2}\)

Dấu"="xảy ra \(\Leftrightarrow a=b=\frac{1}{2}\)

Cách 3:Làm tắt thui ạ

Đặt \(P=\left(a+\frac{1}{b}\right)^2+\left(b+\frac{1}{a}\right)^2\)

\(\left(a+\frac{1}{b}\right)^2+\left(b+\frac{1}{a}\right)^2=a^2+\frac{2a}{b}+\frac{1}{b^2}+b^2+\frac{2b}{a}+\frac{1}{a^2}\ge2ab+\frac{2}{ab}+4\)

\(P\ge2\left(ab+\frac{1}{ab}\right)+4\)

\(P\ge2\left(ab+\frac{1}{16ab}+\frac{15}{16ab}\right)+4\)

giống cách 2 rồi làm nốt