1 , are not doing

2 , are sleeping

3 , goes

4 , is walking - has walked

5 , likes to play

6 , plays

7 , is

8 , isn't playìng

9 , ís teaching

1 , a

2 , c

3 , b

4 , c

3000kg Nhớ **** cho mình

Mark the letter A, B, C, or D on your answer sheet to indicate the word(s) CLOSEST in meaning to the underlined word(s) in each of the following questions.

The whole village was wiped out in the bombing raids.

A. changed completely

B. cleaned well 

C. destroyed completely

D. removed quickly

14/3 lần

năm ngoái thi nhưng cho cô rùi

Xét hiệu : VT - VP

= \(\dfrac{1}{a}\) + \(\dfrac{1}{b}\) - \(\dfrac{4}{a+b}\) = \(\dfrac{a+b}{ab}\) - \(\dfrac{4}{a+b}\)

= \(\dfrac{\left(a+b\right)^2}{ab.\left(a+b\right)}\) - \(\dfrac{4ab}{ab.\left(a+b\right)}\)

= \(\dfrac{a^2+2ab+b^2-4ab}{ab.\left(a+b\right)}\) = \(\dfrac{\left(a-b\right)^2}{ab.\left(a+b\right)}\)

Vì a,b > 0 => a.b > 0 ; a + b > 0

=> ab.(a + b) > 0 . Mà ( a - b)² > 0 nên

\(\dfrac{\left(a-b\right)^2}{ab.\left(a+b\right)}\) \(\ge\) 0 hay VT - VP \(\ge\) 0 ( BĐT đúng )

=> \(\dfrac{1}{a}\) + \(\dfrac{1}{b}\) \(\ge\) \(\dfrac{4}{a+b}\)