`| x | < - 5`
`⇒ x` không tồn tại .
`⇒ x ∈ ∅`
Vậy , `x ∈ ∅ .`
( Vì giá trị tuyệt đối của `1` số luôn lớn hơn hoặc bằng `0` )
Đáp án:
Giải thích các bước giải:
Vì giá trị tuyệt đối của x luôn luôn ≥0
Mà giá trị tuyệt đối của x < -5
⇒x ∈ ∅
16. I’ll come and see you before I _______ for the States. A. leave B. will leave C. have left D. shall leave 17. Sam..............to change a light bulb when he................and..................down. A. was trying-slipped- fell B. tried- was slipping- falling C. had been trying- slipped- was falling D. has tried- slips- falls 18. I couldn’t cut the grass because the lawn mower ______ a few days previously. A. broke down B. has been broken C. had broken down D. breaks down 19. I............. for this company for more then thirty years, and I intend to stay here until I.................next year. A. am working- will retire B. am going to work- am retiring C. work- am going to retire D. have been working- retire 20. Since _______, I have heard nothing from him. A. he had left B. he left C. he has left D. he was left 21. The man got out of the car, ________ round to the back and opened the book. A. walking B. walked C. walks D. walk 22. By the time you receive this letter, I ................for the USA. A. have left B. will leave C. will have left D. am leaving 23. While I ……………….along the road, I saw a friend of mine. A. was cycling B. have cycled C. cycled D. am cycling 24. By the time I . . . . . . . . . . this report, I will give you a ring. A. type B. will type C. have typed D. will have typed 25. While my mother .................... a film on TV, my father was cooking dinner. It was March 8th yesterday. A. watched B. was watching C. had watched D. watches 26. When we came, they _________ a meeting in the hall. A. are having B. were having C. have been having D. have had 27. Tom and Mary ______ for Vietnam tomorrow. A. leave B. are leaving C. leaving D. are left 28. Marie ______ the electric guitar in a rock band now. A. has played B. is playing C. plays D. played 29. We ______ to Ireland for our holiday last year. A. go B. had gone C. have gone D. went 30. My girlfriend ______ walking around late at night during the summer. A. loves B. love C. is loving D. loved 31. They ______ very noisy just now; they’re usually so quiet. A. are B. are being C. have been D. were 32. I saw her in May, but ______ her since then. A. haven’t seen B. didn’t see C. don’t see D. won’t see 33. Tomorrow is my friend’s birthday. I ______ some flowers for her. A. will buy B. was buying C. bought D. buy 34. She ______ the same car for fifteen years. A. has driven B. drives C. drove D. is driving 35. Robert ______ ill for three weeks. He’s still in hospital. A. had been B. has been C. is D. was 36. The grocery store ______ at 8 a.m. and ______ at 7 p.m. every day. A. opens – closes B. open – close C. opened – closed D. is opening - is closing
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