Bài 1:

Ta có: \(S=\dfrac{2015}{2016}+\dfrac{2016}{2017}+\dfrac{2017}{2018}+\dfrac{2018}{2019}+\dfrac{2019}{2015}\)

             \(=\left(\dfrac{2015}{2016}+\dfrac{1}{2016}\right)+\left(\dfrac{2016}{2017}+\dfrac{1}{2017}\right)+\left(\dfrac{2017}{2018}+\dfrac{1}{2018}\right)+\left(\dfrac{2018}{2019}+\dfrac{1}{2019}\right)+\dfrac{2015}{2015}+\dfrac{4}{2015}-\dfrac{1}{2016}-\dfrac{1}{2017}-\dfrac{1}{2018}-\dfrac{1}{2019}\)

\(=5+\left(\dfrac{1}{2015}-\dfrac{1}{2016}\right)+\left(\dfrac{1}{2015}-\dfrac{1}{2017}\right)+\left(\dfrac{1}{2015}-\dfrac{1}{2018}\right)+\left(\dfrac{1}{2015}-\dfrac{1}{2019}\right)\)

Vì \(2015< 2016\Rightarrow\dfrac{1}{2015}>\dfrac{1}{2016}\Rightarrow\dfrac{1}{2015}-\dfrac{1}{2016}>0\)

    \(2015< 2017\Rightarrow\dfrac{1}{2015}>\dfrac{1}{2017}\Rightarrow\dfrac{1}{2015}-\dfrac{1}{2017}>0\)

    \(2015< 2018\Rightarrow\dfrac{1}{2015}>\dfrac{1}{2018}\Rightarrow\dfrac{1}{2015}-\dfrac{1}{2018}>0\)

    \(2015< 2019\Rightarrow\dfrac{1}{2015}>\dfrac{1}{2019}\Rightarrow\dfrac{1}{2015}-\dfrac{1}{2019}>0\)

\(\Rightarrow S=5+\left(\dfrac{1}{2015}-\dfrac{1}{2016}\right)+\left(\dfrac{1}{2015}-\dfrac{1}{2017}\right)+\left(\dfrac{1}{2015}-\dfrac{1}{2018}\right)+\left(\dfrac{1}{2015}-\dfrac{1}{2019}\right)>5\)

Ta có: \(1=4\left(a+b\right)+\sqrt{ab}\ge4.2\sqrt{ab}+\sqrt{ab}=9\sqrt{ab}\Leftrightarrow\sqrt{ab}\le\dfrac{1}{9}\Leftrightarrow ab\le\dfrac{1}{81}\)

  \(\Rightarrow\dfrac{1}{ab}\ge\dfrac{1}{\dfrac{1}{81}}=81\)

Dấu "=" xảy ra \(\Leftrightarrow a=b=\dfrac{1}{9}\)

We could visit Vy on the way if we didn't leave a bit too late

Ta có: 

\(\dfrac{2}{5.7}=\dfrac{7-5}{5.7}=\dfrac{1}{5}-\dfrac{1}{7}\)

\(\dfrac{2}{7.9}=\dfrac{9-7}{7.9}=\dfrac{1}{7}-\dfrac{1}{9}\)

..........

\(\dfrac{2}{53.55}=\dfrac{55-53}{53.55}=\dfrac{1}{53}-\dfrac{1}{55}\)

\(\Rightarrow\dfrac{2}{5.7}+\dfrac{2}{7.9}+...+\dfrac{2}{53.55}=\dfrac{1}{5}-\dfrac{1}{7}+\dfrac{1}{7}-\dfrac{1}{5}+...+\dfrac{1}{53}-\dfrac{1}{55}=\dfrac{1}{5}-\dfrac{1}{55}=\dfrac{10}{55}=\dfrac{2}{11}\)

IV. Choose the word or phrase among A, B, C or D that best fits the blank space

     The survey of eating habits was (19)_________ in Ho Chi Minh City by a group of Japanese nutritionists in order to understand the changes of eating environments and habits accompanying with the economic growth after the war in Viet Nam. The surveys were made in 2002 and 2006. In the survey in 2002, the Vietnamese surely took three meals a day without taking any snacks. They mainly took (20)______ like rice, bread, noodles and some vegetables. But the (21) ______of oils and fats and milk products was rare. It is like (22)_______ of Japan in several decades ago.

     In the survey in 2006, the changes in eating habits were observed. (23) _______the rising of their concern on eating, they rarely took food late at night. The variety and frequency of food was increased. The intake (24)______ snacks was also increased. These changes are (25) ______to have been caused by the change in their attitude towards eating due to the change in lifestyle and those changes had been observed in Japan. More rapidly, however, they were in Viet Nam.

19. A. behaved            B. carried                      C. conducted              D. made

20. A. basics                B. proteins                   C. staples food            D. staples

21. A. processes          B. intake                      C. production              D. amount

22. A. that                   B. what                         C. those                       D. which

23. A. Together with   B. Because                  C. Despite                   D. Due to

24. A. in                      B. of                            C. on                           D. with

25. A. thought             B. found                      C. considered              D. regarded

32. The rescue operation which was a success was arranged by a charity organisation.
 

In universities and colleges in English-speaking countries, the primary language of instruction is English.

33.If people in the world spoke the same language, there wouldn’t be problems in communication.

Gọi chiều dài của hình chữ nhật là x (x∈N^*, x>20)

Chiều rộng của hình chữ nhật là x-20 (m)

Vì chu vi hình chữ nhật là 120m nên ta có pt:

   2.( x+x-20) = 120

⇔ 2x-20 = 60

⇔ x = 40 (thoả mãn) ⇒ Chiều rộng là 40-20 = 20 (m)

Diện tích hình chữ nhật là 40.20 = 800 (m²)