Khi sản xuất vỏ lon sữa bò hình trụ, các nhà thiết kế luôn đặt mục tiêu sao cho chi phí nguyên liệu làm vỏ lon là ít nhất, tức là diện tích toàn phần của hình trụ là nhỏ nhất. Muốn thể tích khối trụ đó bằng $V$ và diện tích toàn phần phần hình trụ nhỏ nhất thì bán kính đáy $R$ bằng:

​Bài tập 2: Chọn từ hoặc cụm từ thích hợp hoàn thành những câu sau.

1. He...............................for 25 years before he retired last year.

​A. has taught​B. had been teaching​C. has been teaching​D. was teaching

2. Please send me a postcard as soon as you .......................... in London.

​A. will arrive​B. is going to arrive​C. have arrived​D. arrive

3. I saw John yesterday morning while I.............................home from work.

​A. walked​B. am walking​C. was walking​D. had been walking

​4. As soon as you........................your homework, you will be allowed to go out.

​A. are doing​B. had done C. did D. have done

5. After Mary..............her degree, she intends to work in her father’s company.

​A. will finish​B. finishesn C. is finishing D. will have finished

​6. She has worked as a secretary...........................she graduated fro college.

​A. until​B. while​C. before​D. since

7. Ann ............................for me when I arrived

​A. was waiting​B. waited​C. had waited C. has been waiting

​8. My handbag was stolen...........................we were playing tennis.

​A. after​B. before​C. during​D. while

9. They won’t come home until they.....................everything.

​A. had seen​B. saw​C. were seeing​D. see

​10. Don’t come..........................I have finished lunch.

​A. after​B. as soon as​C. since​D. until