a.

\(A=\dfrac{2013}{x^2}-\dfrac{2}{x}+1=2013\left(\dfrac{1}{x}-\dfrac{1}{2013}\right)^2+\dfrac{2012}{2013}\ge\dfrac{2012}{2013}\)

Dấu "=" xảy ra khi \(x=2013\)

b.

\(B=\dfrac{4x^2+2-4x^2+4x-1}{4x^2+2}=1-\dfrac{\left(2x-1\right)^2}{4x^2+2}\le1\)

\(B_{max}=1\) khi \(x=\dfrac{1}{2}\)

\(B=\dfrac{-2x^2-1+2x^2+4x+2}{4x^2+2}=-\dfrac{1}{2}+\dfrac{\left(x+1\right)^2}{2x^2+1}\ge-\dfrac{1}{2}\)

\(B_{max}=-\dfrac{1}{2}\) khi \(x=-1\)

P/s : Mik nghĩ là \(\left(2x+1\right)^2\)

\(C=x+\dfrac{1}{4x}+\dfrac{x}{\left(2x+1\right)^2}=\left[\dfrac{x}{\left(2x+1\right)^2}+\dfrac{2x+1}{16}+\dfrac{2x+1}{16}+\dfrac{1}{16x}\right]+\dfrac{3}{4}\left(x+\dfrac{1}{4x}\right)-\dfrac{1}{8}\)

AD BĐT AM - GM ta được : \(\dfrac{x}{\left(2x+1\right)^2}+\dfrac{2x+1}{16}+\dfrac{2x+1}{16}+\dfrac{1}{16x}\ge4\sqrt[4]{\dfrac{1}{16^3}}=\dfrac{1}{2}\)

\(x+\dfrac{1}{4x}\ge2\sqrt{\dfrac{1}{4}}=1\) 

Suy ra : \(C\ge\dfrac{1}{2}+\dfrac{3}{4}.1-\dfrac{1}{8}=\dfrac{9}{8}\)

" = " \(\Leftrightarrow x=\dfrac{1}{2}\)

Biểu thức nào em?

a) Ta có: \(P=\left(\dfrac{x^2-2x}{2x^2+8}-\dfrac{2x^2}{8-4x+2x^2-x^3}\right)\left(1-\dfrac{1}{x}-\dfrac{2}{x^2}\right)\)

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

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

\(=\left(\dfrac{\left(x^2-2x\right)\left(x-2\right)}{2\left(x-2\right)\left(x^2+4\right)}+\dfrac{4x^2}{2\left(x-2\right)\left(x^2+4\right)}\right)\cdot\left(1-\dfrac{1}{x}-\dfrac{2}{x^2}\right)\)

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

\(=\dfrac{x^3+x^2+4x}{2\left(x-2\right)\left(x^2+4\right)}\cdot\dfrac{x^2-x-2}{x^2}\)

\(=\dfrac{x\left(x^2+x+4\right)}{2\left(x-2\right)\left(x^2+4\right)}\cdot\dfrac{\left(x-2\right)\left(x+1\right)}{x^2}\)

\(=\dfrac{\left(x^2+x+4\right)\left(x+1\right)}{2x\left(x^2+4\right)}\)

\(y=\dfrac{x+3}{4}+\dfrac{9}{x-1}=\dfrac{x-1}{4}+\dfrac{9}{x-1}+1\)

\(y\ge2\sqrt{\dfrac{9\left(x-1\right)}{4\left(x-1\right)}}+1=4\)

\(y_{min}=4\) khi \(x=7\)

1.

$x(x+2)(x+4)(x+6)+8$

$=x(x+6)(x+2)(x+4)+8=(x^2+6x)(x^2+6x+8)+8$

$=a(a+8)+8$ (đặt $x^2+6x=a$)

$=a^2+8a+8=(a+4)^2-8=(x^2+6x+4)^2-8\geq -8$

Vậy $A_{\min}=-8$ khi $x^2+6x+4=0\Leftrightarrow x=-3\pm \sqrt{5}$

2.

$B=5+(1-x)(x+2)(x+3)(x+6)=5-(x-1)(x+6)(x+2)(x+3)$

$=5-(x^2+5x-6)(x^2+5x+6)$

$=5-[(x^2+5x)^2-6^2]$

$=41-(x^2+5x)^2\leq 41$

Vậy $B_{\max}=41$. Giá trị này đạt tại $x^2+5x=0\Leftrightarrow x=0$ hoặc $x=-5$

3.

Đặt $x+3=a; 7-x=b$ thì $a+b=10$ 

$C=a^4+b^4$

Áp dụng BĐT Bunhiacopxky:

$(a^4+b^4)(1+1)\geq (a^2+b^2)^2$

$\Rightarrow C\geq \frac{(a^2+b^2)^2}{2}$
$(a^2+b^2)(1+1)\geq (a+b)^2=100$

$\Rightarrow a^2+b^2\geq 50$

$\Rightarrow C\geq \frac{50^2}{2}=1250$

Vậy $C_{\min}=1250$

Giá trị này đạt tại $a=b=5\Leftrightarrow x=2$

\(\left|2x-1\right|+3\ge3\Leftrightarrow\dfrac{3+\left|2x-1\right|}{14}\ge\dfrac{3}{14}\)

Dấu \("="\Leftrightarrow2x-1=0\Leftrightarrow x=\dfrac{1}{2}\)

\(\dfrac{-4x^2+4x}{15}=\dfrac{-4x^2+4x-1+1}{15}=\dfrac{-\left(2x-1\right)^2+1}{15}\)

Ta có \(-\left(2x-1\right)^2+1\le1\Leftrightarrow\dfrac{-\left(2x-1\right)^2+1}{15}\le\dfrac{1}{15}\)

Dấu \("="\Leftrightarrow2x-1=0\Leftrightarrow x=\dfrac{1}{2}\)