cho tam giác ABC cân tại A, đường cao AH .Gọi D là chân đường vuông góc kẻ từ H đến AC , i là trung điểm của HD. gọi M là trung điểm của DC .chứng minh rằng MI vuông góc với AH . Chứng minh rằng AI vuông góc với BD

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I. Put the verbs between brackets in their correct Infinitive or Gerund form : 1. The children are busy (collecting ) shells on the beach. 2. She had to ask the boys (to stop) (making) noise. 3. Our teacher has promised (to help) us (preparing) for the exam. 4. I’d love (having) an opportunity of (to meet) you again. 5. He enjoys (listening) to his own voice. 6. Is (boiling) or (fry) the best way of (cooking) this fish? 7. It’s no use your (asking) him (to lend) you any money. 8. She stopped (talking) as if waiting for him (to speak). 9. My uncle used to (show) me where (to go) and what (to buy). 10. He suggested (start) a new business. 11. We had better (visit) her in hospital. 12. It's very easy to do, so stop (worry)! 13. Could you please stop (making) so much noise? 14. The phone rang while Ann was having dinner. She didn't answer it. She just carried on (eat). 15. The thief got into the house because I forgot (close) the window. 16. I've arranged (play) tennis tomorrow afternoon. 17. One day I'd like to learn (fly) an aero plane. 18. Why do you keep on (look) at me like that? 19. I don't enjoy (write) letters. 20. Why hasn't Sue arrived yet? She promised not (be) late. 21. Jill has decided not (buy) a car. 22. I can't help (wonder) why he did that. 23. He advised us (book) our tickets in advance. 24. Do you remember (dance) on the beach? 25. I look forward to (hear) from you. 26. They allowed us ( play) in the garden. 27. She reminded me (post) the letter. 28. I am sorry (inform) you that you haven't passed the exam. 29. You had better (explain) the rules to her. 30. I can't get used to (live) alone.