\(\left(a^2+\frac{b^2}{4}+\frac{9}{4}+ab-3a-\frac{3}{2}b\right)+\frac{3}{4}\left(b^2-2b+1\right)-\frac{9}{4}-\frac{3}{4}+2013\\ \)

\(\left(a+\frac{b-3}{2}\right)^2+\frac{3}{4}\left(b-1\right)^2+2013-3\)

GTNN=2010

Khi b=1 và a= 1

Hóa ra OLM vẫn còn ADMIN

Lời giải:

Ta có:
\(4M=4a^2+4ab+4b^2-12a-12b+8052\)

\(=(4a^2+4ab+b^2)+3b^2-12a-12b+8052\)

\(=(2a+b)^2-6(2a+b)+9+3b^2-6b+8043\)

\(=[(2a+b)^2-6(2a+b)+9]+3(b^2-2b+1)+8040\)

\(=(2a+b-3)^2+3(b-1)^2+8040\)

\(\geq 0+3.0+8040=8040\)

\(\Rightarrow M\geq \frac{8040}{4}=2010\)

Vậy \(M_{\min}=2010\Leftrightarrow \left\{\begin{matrix} 2a+b-3=0\\ b-1=0\end{matrix}\right. \Rightarrow \left\{\begin{matrix} a=1\\ b=1\end{matrix}\right.\)

Lời giải:

a)

Ta có \(x(x+1)+5=x^2+x+5=\left(x+\frac{1}{2}\right)^2+\frac{19}{4}\)

Vì \(\left(x+\frac{1}{2}\right)^2\geq 0\forall x\in\mathbb{R}\Rightarrow x(x+1)+5\geq 0+\frac{19}{4}=\frac{19}{4}\)

Do đó \((x^2+x+5)_{\min}=\frac{19}{4}\Leftrightarrow x=\frac{-1}{2}\)

b)

\(M=a^2+ab+b^2-3a-3b+2013\)

\(\Rightarrow 2M=2a^2+2ab+2b^2-6a-6b+4026\)

\(\Leftrightarrow 2M=(a+b-2)^2+(a-1)^2+(b-1)^2+4020\)

Thấy \(\left\{\begin{matrix} (a+b-2)^2\geq 0\\ (a-1)^2\geq 0\\ (b-1)^2\geq 0\end{matrix}\right.\Rightarrow 2M\geq 4020\Rightarrow M\geq 2010\)

Vậy \(M_{\min}=2010\Leftrightarrow a=b=1\)

\(VT=\frac{3a}{1+b^2}+\frac{3b}{1+c^2}+\frac{3c}{1+a^2}+\frac{1}{1+b^2}+\frac{1}{1+c^2}+\frac{1}{1+a^2}\)

Ta tách VT=A+B và xét

\(A=\frac{3a}{1+b^2}+\frac{3b}{1+c^2}+\frac{3c}{1+a^2}=\text{∑}\left(3a-\frac{3ab^2}{1+b^2}\right)\ge\text{∑}\left(3a-\frac{3ab}{2}\right)\)

\(B=\frac{1}{1+b^2}+\frac{1}{1+c^2}+\frac{1}{1+a^2}=\text{∑}\left(1-\frac{b^2}{1+b^2}\right)\ge\text{∑}\left(1-\frac{b}{2}\right)\)

\(\Rightarrow VT=A+B=3+\frac{5}{2}\left(a+b+c\right)-\frac{3}{2}\text{∑}ab=\frac{5}{2}\left(a+b+c\right)-\frac{3}{2}\ge\frac{15}{2}-\frac{3}{2}=6\)

(Do \(a+b+c\ge\sqrt{3\left(ab+bc+ca\right)}=3\))

Dấu = khi a=b=c=1

2 + 2 =22

5555555555555 = 5 x 5 x ........

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

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

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

Tương tự ta có: \(\frac{1+3b}{1+c^2}=3b+1-\frac{c}{2}-\frac{3bc}{2}\)(2); \(\frac{1+3c}{1+a^2}=3c+1-\frac{a}{2}-\frac{3ca}{2}\)(3)

Cộng theo vế của 3 BĐT (1), (2), (3), ta được: \(\frac{1+3a}{1+b^2}+\frac{1+3b}{1+c^2}+\frac{1+3c}{1+a^2}\)\(\ge3\left(a+b+c\right)-\frac{a+b+c}{2}-\frac{3\left(ab+bc+ca\right)}{2}+3\)

\(=\frac{5\left(a+b+c\right)}{2}-\frac{3\left(ab+bc+ca\right)}{2}+3\)

\(\ge\frac{5.\sqrt{3\left(ab+bc+ca\right)}}{2}-\frac{3.3}{2}+3=\frac{15}{2}-\frac{9}{2}+3=6\)

Đẳng thức xảy ra khi a = b = c = 1

Các bạn giúp mk nhanh vs aaaaaasắp đến hạn nộp rồi

Bài làm:

Ta có: \(P=\frac{4}{a}+\frac{4}{b}+3a+3b-2\)

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

Áp dụng bất đẳng thức Cauchy ta được:

\(P\ge2\sqrt{\frac{4}{a}.a}+2\sqrt{\frac{4}{b}.b}+2.4-2\)

\(=4+4+8-2=14\)

Dấu "=" xảy ra khi: \(a=b=2\)

Vậy Min(P) = 14 khi a=b=2

Ta có: \(P=\left(\frac{4}{a}+4a\right)+\left(\frac{4}{b}+4b\right)-a-b-2\)

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

Áp dụng bất đẳng thức Cô-si cho phương trình \(a+\frac{1}{a}\)

Ta có: \(\frac{1}{a}+a\ge2\sqrt{\frac{1}{a}.a}=2\)

Áp dụng bất đẳng thức Cô-si cho phương trình \(b+\frac{1}{b}\)

Ta có: \(\frac{1}{b}+b\ge2\sqrt{\frac{1}{b}.b}=2\)

Vì \(a+b\ge4\)nên:

       \(P\ge4.2+4.2-4-2\)

\(\Leftrightarrow P\ge4.3-2\)

\(\Leftrightarrow P\ge10\)

Dấu bằng sảy ra khi và chỉ khi: \(\hept{\begin{cases}a=\frac{1}{a}\\b=\frac{1}{b}\\a+b=4\end{cases}}\)\(\Leftrightarrow\)\(\hept{\begin{cases}a^2=1\\b^2=1\\a+b=4\end{cases}}\)\(\Leftrightarrow\)\(\hept{\begin{cases}a=1\\b=1\\a+b=4\end{cases}}\)

                                  \(\Rightarrow\)\(a,b=\varnothing\)

Vậy \(P_{min}=10\)

khó phết

\(VT=\frac{3a}{1+b^2}+\frac{3b}{1+c^2}+\frac{3c}{1+a^2}+\frac{1}{1+b^2}+\frac{1}{1+c^2}+\frac{1}{1+a^2}\)

Ta tách VT = A + b và xét :

\(A=\frac{3a}{1+b^2}+\frac{3b}{1+c^2}+\frac{3c}{1+a^2}=\Sigma\left(3a-\frac{3ab^2}{1+b^2}\right)\ge\Sigma\left(3a-\frac{3ab}{2}\right)\)\(B=\frac{1}{1+b^2}+\frac{1}{1+c^2}+\frac{1}{1+a^2}=\Sigma\left(1-\frac{b^2}{1+b^2}\right)\ge\Sigma\left(1-\frac{b}{2}\right)\)

\(\Rightarrow VT=A+B=3+\frac{5}{2}\left(a+b+c\right)-\frac{3}{2}\Sigma ab=\frac{5}{2}\left(a+b+c\right)-\frac{3}{2}\ge\frac{15}{2}-\frac{3}{2}=6\)( Do \(a+b+c\ge\sqrt{3\left(ab+bc+ca\right)=3}\))

Dấu = khi a = b = c = 1 .

Yuzuri Yukari:copy câu trả lời của tôi