Lời giải:

$P=4a^2+b^2+c^2+4ab+4ac+2bc=(2a+b+c)^2=(-1)^2=1$

\(P=\dfrac{4ab}{a+2b}+\dfrac{9ca}{a+4c}+\dfrac{4bc}{b+c}\)

\(P=\dfrac{4abc}{ac+2bc}+\dfrac{9abc}{ab+4bc}+\dfrac{4abc}{ab+ac}\)

\(P=abc\left(\dfrac{4}{ac+2bc}+\dfrac{9}{ab+4bc}+\dfrac{4}{ab+ac}\right)\)

\(P\ge abc.\dfrac{\left(2+3+2\right)^2}{ac+2bc+ab+4bc+ab+ac}\)

\(P\ge abc.\dfrac{49}{2ab+6bc+2ca}\)

\(P\ge abc.\dfrac{49}{7abc}\) (vì \(2ab+6bc+2ca=7abc\))

\(P\ge7\)

Dấu "=" xảy ra \(\Leftrightarrow\left\{{}\begin{matrix}\dfrac{2}{ac+2bc}=\dfrac{3}{ab+4bc}=\dfrac{2}{ab+ac}\\2ab+6bc+2ca=7abc\end{matrix}\right.\)

\(\dfrac{2}{ac+2bc}=\dfrac{2}{ab+ac}\) \(\Leftrightarrow2b=a\)

Có \(\dfrac{3}{ab+4bc}=\dfrac{2}{ab+ac}\) 

\(\Leftrightarrow\dfrac{3}{2b^2+4bc}=\dfrac{2}{2b^2+2bc}\) 

\(\Leftrightarrow3b^2+3bc=2b^2+4bc\)

\(\Leftrightarrow b^2=bc\Leftrightarrow b=c\)

\(\Rightarrow a=2b=2c\)

Lại có \(2ab+6bc+2ca=7abc\) \(\Rightarrow4b^2+6b^2+4b^2=14b^3\)

\(\Leftrightarrow b=1\)

\(\Leftrightarrow\left(a,b,c\right)=\left(2,1,1\right)\)

Vậy \(min_P=7\)
 

1 . 

Từ gt : \(2ab+6bc+2ac=7abc\)và \(a,b,c>0\)

Chia cả hai vế cho abc > 0 

\(\Rightarrow\frac{2}{c}+\frac{6}{a}+\frac{2}{b}=7\)

Đặt \(x=\frac{1}{a},y=\frac{1}{b},z=\frac{1}{c}\Rightarrow\hept{\begin{cases}x,y,z>0\\2z+6x+2y=7\end{cases}}\)

Khi đó : \(C=\frac{4ab}{a+2b}+\frac{9ac}{a+4c}+\frac{4bc}{b+c}\)

\(=\frac{4}{2x+y}+\frac{9}{4x+z}+\frac{4}{y+z}\)

\(\Rightarrow C=\frac{4}{2x+y}+2x+y+\frac{9}{4x+z}+4x+z+\frac{4}{y+z}+y+z\)\(-\left(2x+y+4x+z+y+z\right)\)

\(=\left(\frac{2}{\sqrt{x+2y}}-\sqrt{x+2y}\right)^2+\left(\frac{3}{\sqrt{4x+z}}-\sqrt{4x+z}\right)^2\)\(+\left(\frac{2}{\sqrt{y+z}}-\sqrt{y+z}\right)^2+17\ge17\)

Khi \(x=\frac{1}{2},y=z=1\)thì \(C=17\)

Vậy GTNN của C là 17 khi a =2; b =1; c = 1

2 . 

Áp dụng bất đẳng thức Cauchy ta có :\(1+b^2\ge2b\)nên 

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

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

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

Tương tự ta có:

\(\frac{b+1}{1+c^2}\ge b+1-\frac{bc+c}{2}\left(2\right)\)

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

Cộng vế theo vế (1), (2) và (3) ta được: 

\(\frac{a+1}{1+b^2}+\frac{b+1}{1+c^2}+\frac{c+1}{1+a^2}\ge3+\frac{a+b+c-ab-bc-ca}{2}\left(^∗\right)\)

Mặt khác : \(3\left(ab+bc+ca\right)\le\left(a+b+c\right)^2=9\)

\(\Rightarrow\frac{a+b+c-ab-bc-ca}{2}\ge0\)

Nên \(\left(^∗\right)\) \(\Leftrightarrow\frac{a+1}{1+b^2}+\frac{b+1}{1+c^2}+\frac{c+1}{1+a^2}\ge3\left(đpcm\right)\)

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

Chúc bạn học tốt !!!

Lời giải:

\(A=\frac{(bc)^3+(2ac)^3+(2ab)^3}{8a^2b^2c^2}=\frac{(bc)^3+(2ac+2ab)^3-3.2ac.2ab(2ac+2bc)}{8a^2b^2c^2}\)

\(=\frac{(bc)^3+(-bc)^3+12a^2b^2c^2}{8a^2b^2c^2}=\frac{12}{8}=1,5\)

Đáp án A

Từ bảng biến thiên em thấy  P min = P − 2 + 10 4 = 2 10 − 3 2