Hướng dẫn: Tâm đường tròn ngoại tiếp tam giác đều ABC là trực tâm của tam giác ABC.
Các bước giải:
+Từ A hạ đường cao AH xuống BC, ta có: HB = HC = $\frac{BC}{2}$ = $\frac{a}{2}$
+Xét Δ ABH có : AH² + HC² = AC² (Pitago)
⇔ AH² + $\frac{a}{2}$² = a²
⇔ AH² = $\frac{3a²}{4}$
⇒ AH = $\frac{a\sqrt[]{3}}{2}$ (do AH > 0)
+Tâm đường tròn là :
R = $\frac{2}{3}$.AH = $\frac{a\sqrt[]{3}}{3}$.
+Kết luận...
Đáp án:
– Tam giác đều: Tâm của đường tròn ngoại tiếp tam giác trùng với trọng tâm, trực tâm và tâm đường tròn nội tiếp tam giác.
_ Bán kính đường tròn ngoại tiếp tam giác đều là: R=$\frac{2}{3}$ AH=$\frac{a√3}{2}$
( Với AH là đường cao hạ từ A)
Làm hộ mình 1 câu ở lv3 và 1 câu ở lv4 giúp mình với
Cho 11g hỗn hợp Al và Fe tác dụng vừa đủ với dung dịch H2SO4 loãng thu được dd X và 8,96 lít khí H2 (đktc). a, Tính % khối lượng mỗi kim loại có trong hỗn hợp ban đầu . b, Cho từ từ dd NaOH 2M vào dd X. Tính Vdd NaOH để thu đc lượng kết tủa lớn nhất, bé nhất. (Làm hộ mjfnh câu b nha)
Trình bày cảm nhận của em về nhân vật phương định trong đoạn trích từ vắng lặng đến phát sợ đến lao và rít vô hình trên đầu (giúp mình với)
Làm ơn giúp mình với!!! Mình cần gấp sắp nộp rồi . Cảm ơn trước ạ
11. He has twisted his ankle, so he can't play tennis this afternoon. => (NOT) Had _____________, he would have been able to play. * 12. We didn't follow the directions, that's why we got lost. => (WOULD) We ______________________ if we had followed the directions. * 13. You should read more to improve your vocabulary; that's what I'd do. => (WERE) If ______________________ read more to improve my vocabulary. * 14. Jenny can get a puppy only if she promises to take care of it. => (LONG) Jenny can get a puppy ____________________ to take care of it. * 15. I wouldn't mind being transferred to another city if they offered me a higher salary. => (CONDITION) I wouldn't mind being transferred to another city _____________________________ me a higher salary. * 16. Fortunately, they were wearing seat belts, so nobody was seriously injured. => (NOT) If ______________________ seat belts, they could have been seriously injured. * 17. You can borrow my camera providing that you handle with care. => (AS) You can borrow my camera ________________________ care. * 18. Provided she passed her exam, her parents will let her go to Italy for the holdays. => (ON) _________________her exam, her parents will let her go to Italy for the holdays. 19. Should you ever go to Colombia, visit the Museum of Gold in Bogota. => (happen) If _____________to Colombia, visit the Museum of Gold in Bogota. * 20. I think that you shouldn't drink so much coffee. => (WOULDN'T) If __________________ so much coffee.
Giúp mình làm và giải thích với ạ! Mình cảm ơn
V. Read the following passage and choose the item (a, b, c, or d) that best answers each of the questions about it. Legends are stories about the past. Some of the stories come from ancient history. But not all legends are ancient. There are modern legends too. They are often called urban legends, because the stories happen in modern cities. Ten years ago, there was a rumor that there were crocodiles under New York. The rumor was soon a newspaper story, and the newspaper story was soon a legend. So, today, many people in New York believe that there are crocodiles under the ground there! rumor (n): tin đồn 27. What does the word ‘legends’ in line 1 mean? a. stories b. history c. people d. newspapers 28. Which of the following does the word ‘they’ in line 3 refers to? a. stories about the past b. modem legends c. modern cities d. people 29. Another name of ‘modern legends’ is ______ . a. urban legends b. modern cities c. rumors d. crocodiles 30. We can replace the word ‘believe’ in line 7 with _____ . a. ‘know’ b. ‘say’ c. ‘think’ d. ’appear’ 31. Which of the following is true? a. All legends are about the past. b. Modern people still believe in legends. c. There are crocodiles under New York. d. Now many New Yorkers don’t believe that there are crocodiles under the ground of New York.
Nhanh giúp mình với ak cảm ơn rất nhiều
11. He has twisted his ankle, so he can't play tennis this afternoon. => (NOT) Had _____________, he would have been able to play. * 12. We didn't follow the directions, that's why we got lost. => (WOULD) We ______________________ if we had followed the directions. * 13. You should read more to improve your vocabulary; that's what I'd do. => (WERE) If ______________________ read more to improve my vocabulary. * 14. Jenny can get a puppy only if she promises to take care of it. => (LONG) Jenny can get a puppy ____________________ to take care of it. * 15. I wouldn't mind being transferred to another city if they offered me a higher salary. => (CONDITION) I wouldn't mind being transferred to another city _____________________________ me a higher salary. * 16. Fortunately, they were wearing seat belts, so nobody was seriously injured. => (NOT) If ______________________ seat belts, they could have been seriously injured. * 17. You can borrow my camera providing that you handle with care. => (AS) You can borrow my camera ________________________ care. * 18. Provided she passed her exam, her parents will let her go to Italy for the holdays. => (ON) _________________her exam, her parents will let her go to Italy for the holdays. 19. Should you ever go to Colombia, visit the Museum of Gold in Bogota. => (happen) If _____________to Colombia, visit the Museum of Gold in Bogota. * 20. I think that you shouldn't drink so much coffee. => (WOULDN'T) If __________________ so much coffee.dịch
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