đặt x² - x + 13 = a² 

4x² - 4x + 52 = 4a²

( 4x² - 4x + 1 ) - 4a² = -51

( 2x - 1 )² - ( 2a )² = -51

( 2x - 1 - 2a ) ( 2x - 1 + 2a ) = -51

từ đó lập bảng => ...

tìm các cặp số nguyên dương x,y sao cho x^2 +x+13=y

\(x^3-x^2-14x+24\)

\(=x^3-2x^2+x^2-2x-12x+24\)

\(=x^2\left(x-2\right)+x\left(x-2\right)-12\left(x-2\right)\)

\(=\left(x-2\right)\left(x^2+x-12\right)\)

\(=\left(x-2\right)\left(x^2+4x-3x-12\right)\)

\(=\left(x-2\right)\left[x\left(x+4\right)-3\left(x+4\right)\right]\)

\(=\left(x-2\right)\left(x+4\right)\left(x-3\right)\)

Ta có:\(x^3-x^2-14x+24=\left(x^3-2x^2\right)+\left(x^2-2x\right)-\left(12x-24\right)\)

                                                    \(=x^2\left(x-2\right)+x\left(x-2\right)-12\left(x-2\right)\)

                                                    \(=\left(x-2\right)\left(x^2+x-12\right)\)

                                                    \(=\left(x-2\right)\left(x^2-3x+4x-12\right)\)

                                                    \(=\left(x-2\right)\left[x\left(x-3\right)+4\left(x-3\right)\right]\)

                                                    \(=\left(x-2\right)\left(x+4\right)\left(x-3\right)\)

a) \(\frac{3\left(2x+1\right)}{4}-\frac{5x+3}{6}+\frac{x+1}{3}=x+\frac{7}{12}\)

\(\frac{3.3\left(2x+1\right)}{12}-\frac{2\left(5x+3\right)}{12}+\frac{4\left(x+1\right)}{12}=\frac{12x+7}{12}\)

\(18x+9-10x-6+4x+4=12x+7\)

\(0x=0\) ( vô số nghiệm )

Vậy x \(\in\)R

b) ĐKXĐ :  x \(\ne\)-1;-3;-5;-7

\(\frac{1}{x^2+4x+3}+\frac{1}{x^2+8x+15}+\frac{1}{x^2+12x+35}=\frac{3}{16}\)

\(\frac{1}{\left(x+1\right)\left(x+3\right)}+\frac{1}{\left(x+3\right)\left(x+5\right)}+\frac{1}{\left(x+5\right)\left(x+7\right)}=\frac{3}{16}\)

\(\frac{1}{2}\left(\frac{1}{x+1}-\frac{1}{x+3}+\frac{1}{x+3}-\frac{1}{x+5}+\frac{1}{x+5}-\frac{1}{x+7}\right)=\frac{3}{16}\)

\(\frac{1}{x+1}-\frac{1}{x+7}=\frac{3}{8}\)

\(\left(x+1\right)\left(x+7\right)=16\)

Ta thấy x+1 và x+7 là 2 số cách nhau 6 đơn vị . Mà x + 1 < x + 7

\(\Rightarrow\)\(\hept{\begin{cases}x+1=2\\x+7=8\end{cases}\Rightarrow x=1}\)

hoặc \(\hept{\begin{cases}x+1=-2\\x+7=-8\end{cases}}\Rightarrow\hept{\begin{cases}x=-3\\x=-15\end{cases}}\)( loại )

Vậy x = 1

Sai đề không?

đúng đề mà , bài mình vừa thi xong đó

\(x^2-3x+2\)

\(=x^2-2x-x+2\)

\(=x\left(x-2\right)-\left(x-2\right)\)

\(=\left(x-2\right)\left(x-1\right)\)

Để \(f\left(x\right)=\left(x^4+ax^4+bx-1\right)⋮\left(x^2-3x+2\right)\)thì :

\(f\left(x\right)=\left(x^4+ax^4+bx-1\right)=\left(x^2-3x+2\right)\cdot Q\)

\(\Leftrightarrow x^4+ax^4+bx-1=\left(x-2\right)\left(x-1\right)\cdot Q\)

Vì đẳng thức trên đúng với mọi x, do đó :

+) Đặt x = 2 ta có pt :

\(2^4+a\cdot2^4+b\cdot2-1=\left(2-2\right)\left(2-1\right)\cdot Q\)

\(\Leftrightarrow16a+2b+15=0\)

\(\Leftrightarrow16a+2b=-15\)(1)

+) Đặt x = 1 ta có pt :

\(1^4+a\cdot1^4+b\cdot1-1=\left(1-2\right)\left(1-1\right)\cdot Q\)

\(\Leftrightarrow a+b=0\)

\(\Leftrightarrow a=-b\)(2)

Thay (2) vào (1) ta có :

\(16\cdot\left(-b\right)+2b=-15\)

\(\Leftrightarrow-14b=-15\)

\(\Leftrightarrow b=\frac{15}{14}\)

\(\Rightarrow a=\frac{-15}{14}\)

Vậy....

Ta có: \(abc=1\Leftrightarrow\hept{\begin{cases}ab=\frac{1}{c}\\bc=\frac{1}{a}\\ca=\frac{1}{b}\end{cases}}\)

\(abc=1\Leftrightarrow\sqrt[3]{abc}=1\)

Áp dụng BĐT AM-GM ta có:\(1=\sqrt[3]{abc}\le\frac{a+b+c}{3}\Leftrightarrow a+b+c\ge3\)

Dấu " = " xảy ra \(\Leftrightarrow a=b=c=1\)

\(\left(a+b\right)\left(b+c\right)\left(c+a\right)\ge4\left(a+b+c-1\right)\)

\(\Leftrightarrow\)\(a^2b+ab^2+a^2c+ac^2+b^2c+cb^2+2abc+4\ge4\left(a+b+c\right)\)

\(\Leftrightarrow\frac{a}{c}+\frac{b}{c}+\frac{a}{b}+\frac{c}{b}+\frac{b}{a}+\frac{c}{a}+6\ge4\left(a+b+c\right)\)

\(\Leftrightarrow\frac{a+b}{c}+\frac{a+c}{b}+\frac{b+c}{a}+6\ge4\left(a+b+c\right)\)

\(\Leftrightarrow\frac{a+b+c}{c}+\frac{a+c+b}{b}+\frac{a+b+c}{a}+3\ge4\left(a+b+c\right)\)

\(\Leftrightarrow\left(a+b+c\right)\left(\frac{1}{a}+\frac{1}{b}+\frac{1}{c}\right)+3\ge4\left(a+b+c\right)\)

\(\Leftrightarrow\frac{1}{a}+\frac{1}{b}+\frac{1}{c}+\frac{3}{a+b+c}\ge4\)(1)

Ta chứng mĩnh BĐT phụ

Với a,b,c > thì \(\frac{1}{a}+\frac{1}{b}+\frac{1}{c}\ge\frac{9}{a+b+c}\)

Thật vậy.

Áp dụng BĐT AM-GM ta có:

\(\frac{1}{a}+\frac{1}{b}+\frac{1}{c}\ge\frac{3}{\sqrt[3]{abc}}\ge\frac{3}{\frac{a+b+c}{3}}=\frac{9}{a+b+c}\)

Dấu " = " xảy ra \(\Leftrightarrow a=b=c\)

Áp dụng \(\frac{1}{a}+\frac{1}{b}+\frac{1}{c}+\frac{3}{a+b+c}\ge\frac{9}{a+b+c}+\frac{3}{a+b+c}=\frac{12}{3}=4\)(2)

Từ (1) và  (2)

=> \(\left(a+b\right)\left(b+c\right)\left(c+a\right)\ge4\left(a+b+c-1\right)\)

Dấu " = " xảy ra \(\Leftrightarrow a=b=c=1\)

Bạn ơi, tại sao \(\frac{9}{a+b+c}+\frac{3}{a+b+c}=\frac{12}{3}\) được hả bạn?

\(\left(x^2+2x\right)\left(x^2+2x-2\right)=3\)

\(\Leftrightarrow x^4+4x^3+2x^2-4x=3\)

\(\Leftrightarrow x^4+4x^3+2x^2-4x-3=3-3\)

\(\Leftrightarrow x^4+4x^3+2x^2-4x-3=0\)

\(\Leftrightarrow\left(x+1\right)^2\left(x+3\right)\left(x-1\right)=0\)

Dễ rồi, tự làm nốt đi

cảm ơn bạn nhé

bạn là fan của Cris phải ko

Áp dụng BĐT Cauchy-Schwarz dạng Engel,ta có:

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

Dấu "=" xảy ra \(\Leftrightarrow\hept{\begin{cases}a+b=1\\\frac{a}{a+1}=\frac{b}{b+1}\end{cases}}\Leftrightarrow\hept{\begin{cases}a+b=1\\ab+a=ab+b\end{cases}}\Leftrightarrow a=b=\frac{1}{2}\)

Vậy ...