a/ Ta có \(\dfrac{\left(a+b\right)^2}{4}\ge ab\Rightarrow\left(a+b\right)^2\ge4\Rightarrow a+b\ge2\)

\(\left(a+1\right)\left(b+1\right)=ab+\left(a+b\right)+1=a+b+2\ge2+2=4\) (đpcm)

Dấu "=" xảy ra khi \(a=b=1\)

b/ Áp dụng BĐT \(ab\le\dfrac{\left(a+b\right)^2}{4}\Rightarrow ab\le\dfrac{1}{4}\Rightarrow\dfrac{1}{ab}\ge4\)

Lại áp dụng BĐT: \(x^2+y^2\ge\dfrac{\left(x+y\right)^2}{2}\) cho 2 số dương ta được:\(\left(a+\dfrac{1}{b}\right)^2+\left(b+\dfrac{1}{a}\right)^2\ge\dfrac{1}{2}\left(a+b+\dfrac{1}{a}+\dfrac{1}{b}\right)^2=\dfrac{1}{2}\left(1+\dfrac{1}{ab}\right)^2\ge\dfrac{1}{2}\left(1+4\right)^2=\dfrac{25}{2}\)

Dấu "=" xảy ra khi \(a=b=\dfrac{1}{2}\)

\(\frac{a}{b^3-1}+\frac{b}{a^3-1}=\frac{a}{\left(b-1\right)\left(b^2+b+1\right)}+\frac{b}{\left(a-1\right)\left(a^2+a+1\right)}\)

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

\(=-\frac{a^2+a+1+b^2+b+1}{\left(b^2+b+1\right)\left(a^2+a+1\right)}=-\frac{a^2+b^2+3}{a^2b^2+b^2a+b^2+ba^2+ab+b+a^2+a+1}\)

\(=-\frac{\left(a+b\right)^2-2ab+3}{a^2b^2+ab\left(a+b\right)+a^2+b^2+ab+\left(a+b\right)+1}\)

\(=\frac{2ab-4}{a^2b^2+2ab+\left(a+b\right)^2-2ab+2}=\frac{2\left(ab-2\right)}{a^2b^2+3}\)

Có \(ab+bc+ac=abc\Leftrightarrow\dfrac{1}{a}+\dfrac{1}{b}+\dfrac{1}{c}=1\)

Áp dụng các bđt sau:Với x;y;z>0 có: \(\dfrac{1}{x+y+z}\le\dfrac{1}{9}\left(\dfrac{1}{x}+\dfrac{1}{y}+\dfrac{1}{z}\right)\) và \(\dfrac{1}{x+y}\le\dfrac{1}{4}\left(\dfrac{1}{x}+\dfrac{1}{y}\right)\) 

Có \(\dfrac{1}{a+3b+2c}=\dfrac{1}{\left(a+b\right)+\left(b+c\right)+\left(b+c\right)}\le\dfrac{1}{9}\left(\dfrac{1}{a+b}+\dfrac{2}{b+c}\right)\)\(\le\dfrac{1}{9}.\dfrac{1}{4}\left(\dfrac{1}{a}+\dfrac{1}{b}+\dfrac{2}{b}+\dfrac{2}{c}\right)=\dfrac{1}{36}\left(\dfrac{1}{a}+\dfrac{3}{b}+\dfrac{2}{c}\right)\)

CMTT: \(\dfrac{1}{b+3c+2a}\le\dfrac{1}{36}\left(\dfrac{1}{b}+\dfrac{3}{c}+\dfrac{2}{a}\right)\)

\(\dfrac{1}{c+3a+2b}\le\dfrac{1}{36}\left(\dfrac{1}{c}+\dfrac{3}{a}+\dfrac{2}{b}\right)\)

Cộng vế với vế => \(VT\le\dfrac{1}{36}\left(\dfrac{6}{a}+\dfrac{6}{b}+\dfrac{6}{c}\right)=\dfrac{1}{36}.6\left(\dfrac{1}{a}+\dfrac{1}{b}+\dfrac{1}{c}\right)=\dfrac{1}{6}\)

Dấu = xảy ra khi a=b=c=3

Có \(a+b=2\Leftrightarrow2\ge2\sqrt{ab}\Leftrightarrow ab\le1\)

\(E=\left(3a^2+2b\right)\left(3b^2+2a\right)+5a^2b+5ab^2+2ab\)

\(=9a^2b^2+6\left(a^3+b^3\right)+4ab+5ab\left(a+b\right)+20ab\)

\(=9a^2b^2+6\left(a+b\right)^3-18ab\left(a+b\right)+4ab+5ab\left(a+b\right)+20ab\)

\(=9a^2b^2+48-18ab.2+4ab+5.2.ab+20ab\)

\(=9a^2b^2-2ab+48\)

Đặt \(f\left(ab\right)=9a^2b^2-2ab+48;ab\le1\), đỉnh \(I\left(\dfrac{1}{9};\dfrac{431}{9}\right)\)

Hàm đồng biến trên khoảng \(\left[\dfrac{1}{9};1\right]\backslash\left\{\dfrac{1}{9}\right\}\)

 \(\Rightarrow f\left(ab\right)_{max}=55\Leftrightarrow ab=1\)

\(\Rightarrow E_{max}=55\Leftrightarrow a=b=1\)

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