Xét \(a+b\ge1\Leftrightarrow b\ge1-a\)

Xét \(Q\ge\dfrac{8a^2+1-a}{4a}+\left(1-a\right)^2=\dfrac{8a^2}{4a}+\dfrac{1}{4a}-\dfrac{a}{4a}+1-2a+a^2\)

        \(=2a+\dfrac{1}{4a}-\dfrac{1}{4}+1-2a+a^2\)\(=a^2+\dfrac{1}{4a}+\dfrac{3}{4}\)\(=\left(a^2+\dfrac{1}{8a}+\dfrac{1}{8a}\right)+\dfrac{3}{4}\)

Áp dụng Cosi được \(Q\ge3\sqrt[3]{a^2\cdot\dfrac{1}{8a}\cdot\dfrac{1}{8a}}+\dfrac{3}{4}\)\(=3\sqrt[3]{\dfrac{1}{64}}+\dfrac{3}{4}=\dfrac{3}{4}+\dfrac{3}{4}=\dfrac{3}{2}\) 

Vậy \(Qmin=\dfrac{3}{2}\) khi \(a=b=\dfrac{1}{2}\)

\(\frac{8a^2+b}{4a}+b^2=2a+\frac{b}{4a}+b^2=a+a+\frac{b}{4a}+b^2\)

\(\ge a+1-b+\frac{1-a}{4a}+b^2=a+1-b+\frac{1}{4a}-\frac{1}{4}+b^2\)(do \(a+b\ge1\))

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

\(\ge2\sqrt{a\cdot\frac{1}{4a}}+\left(b-\frac{1}{2}\right)^2+\frac{1}{2}\)

\(\ge2\cdot\frac{1}{2}+\frac{1}{2}=\frac{3}{2}\)

Dấu = khi \(a=b=\frac{1}{2}\)

Từ \(a+b\ge1=>b\ge1-a>0\) ta có:

A = \(\dfrac{8a^2+b}{4a}+b^2\ge\dfrac{8a^2+1-a}{4a}+\left(1-a\right)^2\)

=\(\dfrac{8a^2-a+1+4a^3-8a^2+4a}{4a}=\dfrac{4a^3-4a^2+a+4a^2-4a+1+6a}{4a}\)

= \(\dfrac{a\left(2a-1\right)^2+\left(2a-1\right)^2}{4a}+\dfrac{3}{2}=\dfrac{\left(2a-1\right)^2\left(a+1\right)}{4a}+\dfrac{3}{2}\left(1\right)\)

Vì với a>0 thì\(\dfrac{\left(2a-1\right)^2\left(a+1\right)}{4a}\ge0\)

Dấu = xảy ra khi a=1/2

Nên từ (1) => A\(\ge0+\dfrac{3}{2}\) hay A\(\ge\dfrac{3}{2}\)

Vậy GTNN của A=3/2 khi a=b=1/2

A = \(\dfrac{8a^2+b}{4a}+b^2\)

Ta có: a + b \(\ge\) 1 \(\Leftrightarrow\) b \(\ge\) 1 - a

\(\Rightarrow\) A \(\ge\) \(\dfrac{8a^2+1-a}{4a}+\left(1-a\right)^2\)

\(\Leftrightarrow\) A \(\ge\) 2a + \(\dfrac{1}{4a}\) - \(\dfrac{1}{4}\) + 1 - 2a + a²

\(\Leftrightarrow\) A \(\ge\) a² + \(\dfrac{1}{4a}\) + \(\dfrac{3}{4}\)

\(\Leftrightarrow\) A \(\ge\) a² + \(\dfrac{1}{8a}\) + \(\dfrac{1}{8a}\) + \(\dfrac{3}{4}\)

Áp dụng BĐT Cô-si cho 3 số dương a²; \(\dfrac{1}{8a}\); \(\dfrac{1}{8a}\)

a² + \(\dfrac{1}{8a}\) + \(\dfrac{1}{8a}\) \(\ge\) 3\(\sqrt[3]{\dfrac{a^2}{64a^2}}\) = 3\(\sqrt[3]{64}\) = 3.4 = 12

\(\Leftrightarrow\) a² + \(\dfrac{1}{8a}\) + \(\dfrac{1}{8a}\) + \(\dfrac{3}{4}\) \(\ge\) 12 + \(\dfrac{3}{4}\) = \(\dfrac{51}{4}\)

Hay A \(\ge\) a² + \(\dfrac{1}{8a}\) + \(\dfrac{1}{8a}\) + \(\dfrac{3}{4}\) \(\ge\) \(\dfrac{51}{4}\)

Dấu "=" xảy ra \(\Leftrightarrow\) a² = \(\dfrac{1}{8a}\) \(\Leftrightarrow\) 8a³ = 1 \(\Leftrightarrow\) a³ = \(\dfrac{1}{8}\) \(\Leftrightarrow\) a = \(\dfrac{1}{2}\)

và b = 1 - a \(\Leftrightarrow\) b = 1 - \(\dfrac{1}{2}\) = \(\dfrac{1}{2}\)

Vậy Min_A = \(\dfrac{51}{4}\) \(\Leftrightarrow\) a = b = \(\dfrac{1}{2}\)

 Chúc bn học tốt! (ko chắc lắm đâu)

Bạn tham khảo:

Cho hai số thực a;b thay đổi thỏa mãn điều kiện \(a b\ge1\) và \(a>0\) Tìm GTNN của \(A=\frac{8a^2 b}{4a} b^2\) - Hoc24

\(A=\frac{8a^2+b}{4a}+b^2=2a+\frac{b}{4a}+b^2=\left(b^2+\frac{b}{4a}+\frac{a}{2}\right)+\frac{3}{2}a\)

\(\ge3\sqrt[3]{b^2.\frac{b}{4a}.\frac{a}{2}}+\frac{3}{2}a=\frac{3}{2}a+\frac{3}{2}b=\frac{3}{2}\left(a+b\right)\ge\frac{3}{2}\) 

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

\(A=2a+\frac{b}{4a}+b^2=a+a+\frac{b}{4a}+b^2\)

\(A\ge a+1-b+\frac{1-a}{4a}+b^2\)

\(A\ge a+\frac{1}{4a}+b^2-b=a+\frac{1}{4a}+\left(b-\frac{1}{2}\right)^2-\frac{1}{4}\)

\(A\ge a+\frac{1}{4a}-\frac{1}{4}\ge2\sqrt{\frac{a}{4a}}-\frac{1}{4}=\frac{1}{4}\)

\(A_{min}=\frac{1}{4}\) khi \(\left\{{}\begin{matrix}a=\frac{1}{2}\\b=\frac{1}{2}\end{matrix}\right.\)

Lời giải:
Vì $a+b\geq 1\Rightarrow b\geq 1-a; a\geq 1-b$. Do đó:

\(A\geq \frac{8a^2+1-a}{4a}+b^2=2a+\frac{1}{4a}-\frac{1}{4}+b^2\)

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

Áp dụng BĐT AM-GM: \(a+\frac{1}{4a}\geq 1\)

$b^2-b+\frac{1}{4}=(b-\frac{1}{2})^2\geq 0$

Do đó: $A\geq 1+0+\frac{1}{2}=\frac{3}{2}$

Vậy $A_{\min}=\frac{3}{2}$. Dấu "=" xảy ra khi $a=b=\frac{1}{2}$