các tính từ: xinh xẻo, trắng nõn, buồn , nhớ, học trò, nhớ nhung, vui vẻ, yêu thương, đỏ au, vàng chanh, thích, sợ hãi, khó khăn

- Vận tốc trung bình khi xuống dốc: 

          \(v_2=2v_1=2.30=60\left(\dfrac{km}{h}\right)\).

- Quãng đường lên dốc:

          \(\dfrac{s_1}{t_1}=v_1=30\Rightarrow s_1=30t_1\left(km\right)\)

- Quãng đường xuống dốc: 

           \(\dfrac{s_2}{t_2}=v_2=60\Rightarrow s_2=60t_2\left(km\right)\)

- Ta có: \(t_1=\dfrac{t_2}{2}\Rightarrow t_2=2t_1\).

- Vận tốc trung bình trên cả đoạn đường dốc:

      \(v_{tb}=\dfrac{s_1+s_2}{t_1+t_2}=\dfrac{30t_1+60t_2}{t_1+t_2}=\dfrac{30t_1+60.2t_1}{t_1+2t_1}=\dfrac{150t_1}{3t_1}=50\left(\dfrac{km}{h}\right)\)

·         Who always drives his customers away?  A taxi-driver

·         What is the longest word in the English language?  Smiles, because there is a mile between the beginning and the end of it.

·         Why is the letter E so important? Because it is the beginning of everything

·         Where can you always find money? In the dictionary

·         What has ears but can not hear? Corn

·         What has arms but can not hug? Armchair

·         What month do soldiers hate? March

a/

\(a^2+b^2=ab+ba=2ab\)

\(\Rightarrow\left(a-b\right)^2=0\Rightarrow a-b=0\Rightarrow a=b\)

b/ Ta có

\(a^3+b^3+c^3-3abc=\left(a+b+c\right)\left(a^2+b^2+c^2-ab-bc-ca\right)\)

Mà \(a^2+b^2+c^2=ab+bc+ca\)

\(\Rightarrow a^3+b^3+c^3=3abc\)

Theo AM-GM có

\(\dfrac{a^3+b^3+c^3}{3}\ge\sqrt[3]{a^3b^3c^3}=abc\)

\(\Rightarrow a^3+b^3+c^3\ge3abc\) Dấu = xảy ra khi

\(a^3=b^3=c^3\Rightarrow a=b=c\)

`2x^3-16x^2+18x=0`

`<=>2x(x^2-8x+9)=0`

`<=>2x(x^2-8x+16-7)=0`

`<=>2x[(x-4)^2-7]=0`

`<=>` $\left[\begin{matrix} 2x=0\\ (x-4)^2-7=0\end{matrix}\right.$

`<=>` $\left[\begin{matrix} x=0\\ (x-4)^2=7\end{matrix}\right.$

`<=>` $\left[\begin{matrix} x=0\\ x-4=\pm \sqrt{7}\end{matrix}\right.$

`<=>` $\left[\begin{matrix} x=0\\ x=4 \pm \sqrt{7}\end{matrix}\right.$

Vậy `S={0;4+-\sqrt{7}}`