Đặt 6x+7=a Ta có \(\left(a^2-1\right)a^2=72\Leftrightarrow a^4-a^2-72=0\Leftrightarrow\left(a^2+8\right)\left(a^2-9\right)=0\)Mà a^2+8>0 nên \(a^2-9=0\Rightarrow a=+-3\Rightarrow6x+7=+-3\Rightarrow\left[{}\begin{matrix}x=-\frac{2}{3}\\x=-\frac{5}{3}\end{matrix}\right.\)

Ta có : \(\left(6x+6\right)\left(6x+8\right)\left(6x+7\right)^2=72\)

=> \(\left(36x^2+84x+48\right)\left(36x^2+84x+49\right)=72\)

- Đặt \(36x^2+84x+48=a\) ta được phương trình :

\(a\left(a+1\right)=72\)

=> \(a^2+a-72=0\)

=> \(\left(a-8\right)\left(a+9\right)=0\)

=> \(\left[{}\begin{matrix}a=8\\a=-9\end{matrix}\right.\)

- Thay lại \(36x^2+84x+48=a\) vào phương trình trên ta được :

\(\left[{}\begin{matrix}36x^2+84x+48=8\\36x^2+84x+48=-9\end{matrix}\right.\)

=> \(\left[{}\begin{matrix}\left(6x+7\right)^2=9\\\left(6x+7\right)^2=-8\left(vl\right)\end{matrix}\right.\)

=> \(\left[{}\begin{matrix}6x+7=\sqrt{9}\\6x+7=-\sqrt{9}\end{matrix}\right.\)

=> \(\left[{}\begin{matrix}6x=-4\\6x=-10\end{matrix}\right.\)

=> \(\left[{}\begin{matrix}x=-\frac{2}{3}\\x=-\frac{5}{3}\end{matrix}\right.\)

Vậy phương trình có tập nghiệm là \(S=\left\{-\frac{2}{3};-\frac{5}{3}\right\}\)

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đặt 6x+7=a

suy ra (a-1)(a+1)a²=72

(a²-1)a²=72

a⁴-a²+1/4=289/4

(a²-1/2)=289/4

hoặc a²-1/2=17/2

         a²-1/2=-17/2

suy ra hoặc a²=9

                    a²=-8(loại vì a²>=0>-8 với mọi a )

suy ra a=3

            a=-3

hay 6x+7=3 suy ra x=-2/3

       6x+7=-3 suy ra x=-5/3

vậy S={-2/3,-5/3}

a. Làm gọn 1 chút xíu:

\(y=\left(x^{11}+2x^7-3x^5-6x\right)\left(3x^7+6x^2-2\right)\)

\(y'=\left(11x^{10}+14x^6-15x^4-6\right)\left(3x^7+6x^2-2\right)+\left(21x^6+12x\right)\left(x^{11}+2x^7-3x^5-6x\right)\)

b.

 \(y'=5\left(x^4-\dfrac{2}{3x}\right)^4\left(4x^3+\dfrac{2}{3x^2}\right)\Rightarrow y'\left(10\right)=5\left(10^4-\dfrac{2}{30}\right)^4\left(4.10^3+\dfrac{2}{300}\right)=?\)

c.

\(y'=\dfrac{7}{\left(x+1\right)^2}\Rightarrow y'\left(4\right)=\dfrac{7}{25}\)

1: \(\Leftrightarrow2x^2-10x-3x-2x^2=0\)

=>-13x=0

=>x=0

2: \(\Leftrightarrow5x-2x^2+2x^2-2x=13\)

=>3x=13

=>x=13/3

3: \(\Leftrightarrow4x^4-6x^3-4x^3+6x^3-2x^2=0\)

=>-2x^2=0

=>x=0

4: \(\Leftrightarrow5x^2-5x-5x^2+7x-10x+14=6\)

=>-8x=6-14=-8

=>x=1

`1)2x(x-5)-(3x+2x^2)=0`

`<=>2x^2-10x-3x-2x^2=0`

`<=>-13x=0`

`<=>x=0`

___________________________________________________

`2)x(5-2x)+2x(x-1)=13`

`<=>5x-2x^2+2x^2-2x=13`

`<=>3x=13<=>x=13/3`

___________________________________________________

`3)2x^3(2x-3)-x^2(4x^2-6x+2)=0`

`<=>4x^4-6x^3-4x^4+6x^3-2x^2=0`

`<=>x=0`

___________________________________________________

`4)5x(x-1)-(x+2)(5x-7)=0`

`<=>5x^2-5x-5x^2+7x-10x+14=0`

`<=>-8x=-14`

`<=>x=7/4`

___________________________________________________

`5)6x^2-(2x-3)(3x+2)=1`

`<=>6x^2-6x^2-4x+9x+6=1`

`<=>5x=-5<=>x=-1`

___________________________________________________

`6)2x(1-x)+5=9-2x^2`

`<=>2x-2x^2+5=9-2x^2`

`<=>2x=4<=>x=2`

Akai Haruma

Nguyễn Việt Lâm