Khảo sát thực nghiệm một con lắc lò xo gồm vật nhỏ có khối lượng 216 g và lò xo có độ cứng k, dao động dưới tác dụng của ngoại lực F   =   F 0 cos 2 π f t , với   F 0 không đổi và f thay đổi được. Kết quả khảo sát ta được đường biểu diễn biên độ A của con lắc theo tần số f có đồ thị như hình vẽ. Giá trị của k xấp xỉ bằng

A. 13,64 N/m.

B. 12,35 N/m.

C. 15,64 N/m

D. 16,71 N/m.

Read the following passage and mark the letter A, B, C or D on your answer sheet to indicate the correct answer to each of the questions from 43 to 50:

         Basic to any understanding of Canada in the 20 years after the Second World War is the country’s impressive population growth. For every three Canadians in 1945, there were over five in 1966. In September 1966 Canada’s population passed the 20 million mark. Most of his surging growth came from natural increase. The depression of the 1930’s and the war had held back marriages, and the catching-up process began after 1945. The baby boom continued through the decade of the 1950s, producing a population increase of nearly fifteen percent in the five years from 1951 to 1956. This rate of increase had been exceeded only once before settle. Undoubtedly, the good economic conditions of the 1950’s supported a growth in the population, but the expansion also derived from a trend toward earlier marriages and an increase in the average size of families. In 1957 the Canadian birth rate stood at 28 per thousand, one of the highest in the world.

           After the peak year of 1957, the birth rate in Canada began to decline. It continued falling until in 1966 it stood at the lowest level in 25 years. Partly this decline reflected the low level of births during the depression and the war, but it was also caused by changes in Canadian society. Young people were staying at school longer; more women were working; young married couples were buying automobiles or houses before starting families; rising living standards were cutting down the size of families. It appeared that Canada was once more falling in step with the trend toward smaller families that had occurred all through the Western world since the time of the Industrial Revolution.

             Although the growth in Canada’s population had slowed down by 1966 (the increase in the first half of the 1960’s was only nine percent), another large population wave was coming over the horizon. It would be composed of the children who were born during the period of the high birth rate prior to 1957.

The word “surging” in bold is closest in meaning to

A. new

B. extra

C. accelerating

D. surprising

Cho X là axit cacboxylic, Y là amino axit (phân tử có một nhóm – N H 2 ). Đốt cháy hoàn toàn 0,5 mol hỗn hợp gồm X và Y, thu được khí N 2 ; 15,68 lít khí C O 2 (đktc) và 14,4 gam H 2 O . Mặt khác, 0,35 mol hỗn hợp trên phản ứng vừa đủ với dung dịch chứa m gam HCl. Giá trị của m là :

A. 6,39

B. 4,38

D. 10,22.

D. 5,11

Asian games ……….come from many different countries.

A. athletics

B. athletic

C. athlete

D. athletes

Trong số các chất CH₃CH₂OH, CH₃CH₂NH₂, HCOOH, CH₃COOH. Chất có nhiệt độ sôi cao nhất là : 

A. CH₃CH₂OH

B. HCOOH

C. CH₃COOH

D. CH₃CH₂NH₂

Cho lục giác đều ABCDEF tâm O. Hãy chỉ ra các vectơ bằng vectơ AB có điểm đầu và điểm cuối là O hoặc các đỉnh của lục giác.

: - A: "You look nice in that red hat." - B: "_______”

A. I am interesting to hear that.

B. It's nice of you to say so. 

C. Oh, poor me.

D. Am I ? Thanks.