1. What did the people do when you were there

2. I visited Dalat with my parents.

3. What do you think of Tam?

4. The life in countryside is peaceful and more than I expected

1.What did the people do when you were there?

2.I visited Da Lat with my parents.

3.What do you think of Tam?

4.The life in the countryside is more peaceful than I expected.

???

Ta có \(\dfrac{1}{1.3}\)+\(\dfrac{1}{3.5}\)+\(\dfrac{1}{5.7}\)+...+\(\dfrac{1}{49.51}\)

         =\(\dfrac{2}{2}\).(\(\dfrac{1}{1.3}\)+\(\dfrac{1}{3.5}\)+\(\dfrac{1}{5.7}\)+...+\(\dfrac{1}{49.51}\))

         =\(\dfrac{1}{2}\).(\(\dfrac{2}{1.3}\)+\(\dfrac{2}{3.5}\)+\(\dfrac{2}{5.7}\)+...+\(\dfrac{2}{49.50}\))

         =\(\dfrac{1}{2}\).(1-\(\dfrac{1}{3}\)+\(\dfrac{1}{3}-\dfrac{1}{5}+\dfrac{1}{5}-\dfrac{1}{7}+...+\dfrac{1}{49}-\dfrac{1}{51}\))

         =\(\dfrac{1}{2}\).(\(1-\dfrac{1}{51}\))

         =\(\dfrac{1}{2}\).\(\dfrac{50}{51}\)

         =\(\dfrac{25}{51}\)

Ta có: \(\dfrac{1}{1\cdot3}+\dfrac{1}{3\cdot5}+...+\dfrac{1}{49\cdot51}\)

\(=\dfrac{1}{2}\left(\dfrac{2}{1\cdot3}+\dfrac{2}{3\cdot5}+...+\dfrac{2}{49\cdot51}\right)\)

\(=\dfrac{1}{2}\left(1-\dfrac{1}{3}+\dfrac{1}{3}-\dfrac{1}{5}+...+\dfrac{1}{49}-\dfrac{1}{51}\right)\)

\(=\dfrac{1}{2}\left(1-\dfrac{1}{51}\right)\)

\(=\dfrac{1}{2}\cdot\dfrac{50}{51}=\dfrac{25}{51}\)

1. 

- Hợp chất cấu tạo từ 2 nguyên tố: Fe và Cl

- Hợp chất có: 1 nguyên tố Fe và 3 nguyên tố Cl

- \(PTK=56+35,5\text{ x }3=162,5\left(đvC\right)\)

===========

2. 

- Hợp chất cấu tạo từ 3 nguyên tố: Na, C và O

- Hợp chất có: 2 nguyên tố Na, 1 nguyên tố C và 3 nguyên tố O

- \(PTK=23\text{ x }2+12+16\text{ x }3=106\left(đvC\right)\)

==========

3. 

- Hợp chất cấu tạo từ 3 nguyên tố: H, P và O

- Hợp chất có: 2 nguyên tố H, 1 nguyên tố P và 4 nguyên tố O

- \(PTK=1\text{ x }3+31+16\text{ x }4=98\left(đvC\right)\)

==========

4.

- Hợp chất cấu tạo từ 2 nguyên tố: S và O

- Hợp chất có: 1 nguyên tố S và 3 nguyên tố O

- \(PTK=32+16\text{ x }3=80\left(đvC\right)\)

em ơi chưa có bài em nhé, em chưa tải bài lên lám sao mình giúp được 

Dạ đề đây ạ   

Dạ đề đây ạ  

c. \(\left|\dfrac{8}{4}-\left|x-\dfrac{1}{4}\right|\right|-\dfrac{1}{2}=\dfrac{3}{4}\)

\(\Rightarrow\left[{}\begin{matrix}\left|\dfrac{8}{4}-x+\dfrac{1}{4}\right|-\dfrac{1}{2}=\dfrac{3}{4}\\\left|\dfrac{8}{4}+x-\dfrac{1}{4}\right|-\dfrac{1}{2}=\dfrac{3}{4}\end{matrix}\right.\) \(\Rightarrow\left[{}\begin{matrix}\left|\dfrac{9}{4}-x\right|-\dfrac{1}{2}=\dfrac{3}{4}\\\left|\dfrac{7}{4}+x\right|-\dfrac{1}{2}=\dfrac{3}{4}\end{matrix}\right.\)

\(\Rightarrow\left[{}\begin{matrix}\left[{}\begin{matrix}\dfrac{9}{4}-x-\dfrac{1}{2}=\dfrac{3}{4}\\x=\dfrac{9}{4}-\dfrac{1}{2}=\dfrac{3}{4}\end{matrix}\right.\\\left[{}\begin{matrix}\dfrac{7}{4}+x-\dfrac{1}{2}=\dfrac{3}{4}\\-\dfrac{7}{4}-x-\dfrac{1}{2}=\dfrac{3}{4}\end{matrix}\right.\end{matrix}\right.\) \(\Rightarrow\left[{}\begin{matrix}\left[{}\begin{matrix}x=1\\x=\dfrac{7}{2}\end{matrix}\right.\\\left[{}\begin{matrix}x=-\dfrac{1}{2}\\x=-3\end{matrix}\right.\end{matrix}\right.\) \(\Rightarrow\left[{}\begin{matrix}x=1\\x=\dfrac{7}{2}\\x=-3\end{matrix}\right.\) 

Ở nơi x=9/4-1/2 là x-9/4-1/2 nha

a. -1,5 + 2x = 2,5

<=> 2x = 2,5 + 1,5

<=> 2x = 4

<=> x = 2

b. \(\dfrac{3}{2}\left(x+5\right)-\dfrac{1}{2}=\dfrac{4}{3}\)

<=> \(\dfrac{3}{2}x+\dfrac{15}{2}-\dfrac{1}{2}=\dfrac{4}{3}\)

<=> \(\dfrac{9x}{6}+\dfrac{45}{6}-\dfrac{3}{6}=\dfrac{8}{6}\)

<=> 9x + 45 - 3 = 8

<=> 9x = 8 + 3 - 45

<=> 9x = -34

<=> x = \(\dfrac{-34}{9}\)

=2001

Câu 16: A