Đáp án+Giải thích các bước giải:
$A = 1.2 + 2.3 + 3.4 + … + n.(n + 1)\\3A = 1.2.3 + 2.3.(4-1) + 3.4.(5-2) + … + n.(n + 1).[(n+2)-(n-1)]\\3A=1.2.3+2.3.4-1.2.3+3.4.5-2.3.4+...+n.(n+1).(n+2)-(n-1).n.(n+1)\\3A=n.(n+1).(n+2)\\A=\dfrac{n.(n+1).(n+2)}3$
Đáp án:
Giải thích các bước giải:
A = 1.2 + 2.3 + 3.4 + … + n.(n + 1)
3A=1.2.3+2.3.3+3.4.3+...+3n(n+1)
3A=1.2.3+2.3.(4-1)+3.4.(5-2)+...+n(n+1)(n+2-(n+1))
3A=1.2.3+2.3.4+3.4.5+...+n(n+1)(n +2)-(n-1)n(n+1)
3A=n.(n+1)(n+2)
A=(n.(n+1)(n+2))/3
vậy A =(n.(n+1)(n+2))/3
tại sao khối chóp có đáy là hình bình hành S.ABCD nếu có các cạnh bên bằng nhau thì thì đáy phải là tứ giác nội tiếp/ hình chữ nhật v ạ
V. Reoder the words to make meaningful sentences: (2pts) 26. engineer / an / a / brother / is / or / doctor / your ? ->…………..……………….……………………………………………… 27. one / two / living-room / are / bedrooms / house / and / my / there / in. ->……………………..………………………………………………… 28. sister / are / students / those / is / and / my / her / this. ->………………..………………………………………………………… 29. have / any eggs / some meat / I / but / have / don’t / I. -> ……………………………………………………………………………... 30. students / mai’s / many / class / how / there / in / are? -> …………………………………………………………………………...
Mn giúp em với ạ ai xong trước em cho 5 sao
1. His good behaviours made a deep…………….. on me A. expression B. impression C. happiness D. kindness 2. It‟s pouring with rain again! If only I………..an umbrella A. have B. having C. have had D. had 3. “Where do you live, the boy?”, asked the man In reported speech this should be said: A. The man asked the boy Where do you live? B. The man asked the boy Where he lives. C. The man asked the boy Where he lived. D. The man asked the boy Where did he live? 4. Deaf-mutes can ………. speak …………hear. A. not only…but also B. either …or C. neither …nor D. both …and 5. Tam has a wonderful sense of humor. A. He is very able to laugh B. He can laugh easily C. He is very able to see things as amusing D. He is often laughed at 6. We need someone with energy to do this job A. activeness B. intelligence C. strength D. wealth 7. Let‟s try to be adult about this. A. We should try to grow up quickly B. We should act reasonably C. We should act childishly D. We shouldn‟t tell the adult about this. 8. It‟s high time he ……………… . A. starts B. is starting C. will start D. started 9. The children stopped ……………… when I entered the classroom. A. talk B. talking C. talked D. to talk 10. Not only he but also I …………… late for school. A. am B. are C. to be D. is 11. She asked me ……………… the room when she was studying. A. not enter B. not to enter C. enter D. entering 12. I caught a thief ……………… through the key‟s hole. A. looking B. look C. to look D. looked 13. Mr Nam found it ……………… to swim in the sea. A. difficult B. difficulty C. difficultly D. none is correct 14. It is ……………… who took my watch. A. him B. himself C. his D. he 15. I can‟t help …………when I see her slipping on a banana skin. A. to laugh B. laugh C. laughing D. none is correct 16. When I met her, she ………..to her boyfriend. A. was talking B. had talked C. talkes D. is talking
Giúp mình với chiều mình phải nộp rồi
An đếm bắt đầu từ 50, số sau hơn số ngay trước 3 đơn vị. Bình đến bắt đầu từ 20, số sau hơn số trước 5 đơn vị. Nếu cả hai bạn cùng đọc to một lúc số của mình thì số đầu tiên giống nhau là bao nhiêu?
Nhanh ạ ghi đáp án dưới phần bình luận r tl sau ạ.Nhanh giúp ạ
`a/b=c/d` thì: `(7a^2+3ab)/(11a^2-8b^2)=(7c^2+3cd)/(11c^2-8d^2)`
48x8 ko dùng máy tính tính đâu nha tính nhẩm
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