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\(A=a+b+c+\dfrac{3}{a}+\dfrac{9}{2b}+\dfrac{4}{c}\\ A=\left(\dfrac{3a}{4}+\dfrac{3}{a}\right)+\left(\dfrac{b}{2}+\dfrac{9}{2b}\right)+\left(\dfrac{c}{4}+\dfrac{4}{c}\right)+\left(\dfrac{a}{4}+\dfrac{b}{2}+\dfrac{3c}{4}\right)\\ A=\left(\dfrac{3a}{4}+\dfrac{3}{a}\right)+\left(\dfrac{b}{2}+\dfrac{9}{2b}\right)+\left(\dfrac{c}{4}+\dfrac{4}{c}\right)+\dfrac{1}{4}\left(a+2b+3c\right)\\ A\ge2\sqrt{\dfrac{3a}{4}\cdot\dfrac{3}{a}}+2\sqrt{\dfrac{b}{2}\cdot\dfrac{9}{2b}}+2\sqrt{\dfrac{c}{4}\cdot\dfrac{4}{c}}+\dfrac{1}{4}\cdot20\\ A\ge3+3+2+5=13\\ A_{min}=13\Leftrightarrow\left\{{}\begin{matrix}a=2\\b=3\\c=4\end{matrix}\right.\)
\(\frac{c+1}{c+3}\ge\frac{1}{a+2}+\frac{3}{b+4}\ge2\sqrt[]{\frac{3}{\left(a+2\right)\left(b+4\right)}}\) (1)
\(\frac{1}{a+2}+\frac{3}{b+4}\le\frac{c+3-2}{c+3}=1-\frac{2}{c+3}\Rightarrow1-\frac{1}{a+2}\ge\frac{3}{b+4}+\frac{2}{c+3}\)
\(\Rightarrow\frac{a+1}{a+2}\ge\frac{3}{b+4}+\frac{2}{c+3}\ge2\sqrt{\frac{6}{\left(b+4\right)\left(c+3\right)}}\) (2)
\(\frac{1}{a+2}+\frac{3}{b+4}\le1-\frac{2}{c+3}\Rightarrow1-\frac{3}{b+4}\ge\frac{1}{a+2}+\frac{2}{c+3}\)
\(\Rightarrow\frac{b+1}{b+4}\ge\frac{1}{a+2}+\frac{2}{c+3}\ge2\sqrt{\frac{2}{\left(a+2\right)\left(c+3\right)}}\) (3)
Nhân vế với vế (1);(2);(3):
\(\frac{\left(a+1\right)\left(b+1\right)\left(c+1\right)}{\left(a+2\right)\left(b+4\right)\left(c+3\right)}\ge8\sqrt{\frac{36}{\left(a+2\right)^2\left(b+4\right)^2\left(c+3\right)^2}}=\frac{48}{\left(a+2\right)\left(b+4\right)\left(c+3\right)}\)
\(\Rightarrow Q\ge48\Rightarrow Q_{min}=48\) khi \(\left(a;b;c\right)=\left(1;5;3\right)\)
\(P=\text{∑}\frac{a\left(\frac{1}{a}+1+c\right)}{\left(a^3+b^2+c\right)\left(\frac{1}{a}+1+c\right)}\le\frac{\text{∑}\left(1+a+ac\right)}{\left(a+b+c\right)^2}\)
\(\le\frac{3+a+b+c+\frac{\left(a+b+c\right)^2}{3}}{\left(a+b+c\right)^2}\)
\(\le\frac{3+3+\frac{3^2}{3}}{3^2}=1\)
"=" khi a=b=c=1
\(P=\frac{1}{a}+\frac{1}{b}+\frac{4}{c}\ge\frac{\left(1+1+2\right)^2}{a+b+c}=\frac{16}{4}=4\)
1
Áp dụng BĐT Cauchy cho 2 số dương:
4ac=2.b.2c≤2(b+2c2)2≤2(a+b+2c2)2=2.(12)2=12
⇒−4bc≥−12
⇒K=ab+4ac−4bc≥−4bc≥−12
2ab + 6bc + 2ac = 7abc => \(\frac{2}{c}+\frac{6}{a}+\frac{2}{b}=7\)
đặt \(x=\frac{1}{a};y=\frac{1}{b};z=\frac{1}{c}\) => 6x + 2y + 2z = 7; x; y; z > 0
Khi đó, C = \(\frac{4}{\frac{1}{b}+\frac{2}{a}}+\frac{9}{\frac{1}{c}+\frac{4}{a}}+\frac{4}{\frac{1}{c}+\frac{1}{b}}=\frac{4}{2x+y}+\frac{9}{4x+z}+\frac{4}{y+z}\)
AD BĐT Cauchy ta có:
\(\left(\frac{4}{2x+y}+\left(2x+y\right)\right)+\left(\frac{9}{4x+z}+\left(4x+z\right)\right)+\left(\frac{4}{y+z}+\left(y+z\right)\right)\)
\(\ge2\sqrt{4}+2.\sqrt{9}+2.\sqrt{4}=14\)
=> \(\frac{4}{2x+y}+\frac{9}{4x+z}+\frac{4}{y+z}\)+ 7 > 14 => C > 7
Dấu "=" xảy ra <=> a = 2; b = 1; c = 1
Vậy Min C = 7
2ab+6bc+2ac=7abc =>
Đặt => 6x + 2y + 2z = 7; x; y; z > 0
Khi đó C=
TA CÓ:
Dấu “=” xảy raóa=2;b=1;c=1
Vậy c=7
Xong rồi đó bạn hứa cho mik nha
Chỗ kia phải là \(\dfrac{c^4}{b+a+4ba}\) chứ nhỉ? Nếu đúng đề thì bạn nói với mình để mình làm lại nhé. Giờ mình làm theo đề đối xứng trước nhé.
Ta có:
\(P=\dfrac{a^6}{a^2b+a^2c+4a^2bc}+\dfrac{b^6}{b^2a+b^2c+4b^2ca}+\dfrac{c^6}{c^2a+c^2b+4c^2ab}\)
\(\ge\dfrac{\left(a^3+b^3+c^3\right)^2}{a^2b+b^2c+c^2a+ab^2+bc^2+ca^2+4a^2bc+4b^2ca+4c^2ab}\)
\(=\dfrac{9}{\left(a+b+c\right)\left(ab+bc+ca\right)+abc\left(4\left(a+b+c\right)-3\right)}\)
Ta có \(ab+bc+ca\le\dfrac{\left(a+b+c\right)^2}{3}\)
và \(abc\le\dfrac{a^3+b^3+c^3}{3}=1\), đồng thời \(\left(a+b+c\right)^3=a^3+b^3+c^3+3\left(a+b\right)\left(b+c\right)\left(c+a\right)=3+3\left(a+b\right)\left(b+c\right)\left(c+a\right)>\dfrac{27}{64}\)
\(\Leftrightarrow a+b+c>\dfrac{3}{4}\) \(\Leftrightarrow4\left(a+b+c\right)-3>0\). Do đó \(abc\left(4\left(a+b+c\right)-3\right)\le4\left(a+b+c\right)-3\)
Vì vậy \(P\ge\dfrac{9}{\dfrac{\left(a+b+c\right)^3}{3}+4\left(a+b+c\right)-3}\)
Đặt \(a+b+c=t\).
Ta có \(a^3+b^3\ge ab\left(a+b\right)=a^2b+b^2a\). Lập 2 BĐT tương tự rồi cộng theo vế, ta có:
\(2\left(a^3+b^3+c^3\right)\ge a^2b+b^2c+c^2a+ab^2+bc^2+ca^2\)
\(\Leftrightarrow\left(a+b\right)\left(b+c\right)\left(c+a\right)\le6+2abc\le8\) (vì \(abc\le1\))
Do đó \(t^3=3+3\left(a+b\right)\left(b+c\right)\left(c+a\right)\le3+3.8=27\) \(\Leftrightarrow t\le3\)
Vậy \(0< t\le3\)
Ta có \(P\ge\dfrac{9}{\dfrac{t^3}{3}+4t-3}\) \(\ge\dfrac{9}{\dfrac{3^3}{3}+4.3-3}=\dfrac{1}{2}\)
Dấu "=" xảy ra khi \(a=b=c=1\)
Vậy GTNN của P là \(\dfrac{1}{2}\) khi \(a=b=c=1\)