cho tứ diện ABCD có M,N là trung điểm cạnh AB,CD và P là một điểm thuộc cạnh BC (P không là trung điểm của BC) a) XÁc định tứ diện cắt bởi (MNP) b) CM: MN chia đôi thiết diện

2 câu trả lời

Noi MP. Keo dai PN cat D tai E. Noi ME cat AD tai F. Noi FN. Vay thiet dien la MFNP.

b)

a) Ta có: \((MNP)\cap(ABC)=MN\)

\((MNP)\cap(BCD)=PN\)

\(PN\cap BD=E\) khi đó \(E\in BD, BD\subset(ABD)\)

\(\Rightarrow E\in (ABD)\)

\(EM\cap AD=F\)

\(\Rightarrow (MNP)\cap(ABD)=MF\) và \((MNP)\cap(ADC)=FN\)

Khi đó 4 đoạn giao tuyến là: \(MP\), \(PN\), \(MF\), \(FN\).

Vậy thiết diện là tứ diện \(MPNF\).

b) Từ hình vẽ thấy rằng tứ giác \(MPNF\) có đường chéo là \(MN\) nên \(MN\) chia thiết diện ra làm hai.

Câu hỏi trong lớp Xem thêm

giup e voi I/ Supply the correct form of the verbs in brackets 1. You (think) ……………………… collecting stamps costs much money? 2. Every year, my mother (give) ……………………… me a nice doll on my birthday. 3. Jenny says she loves collecting pens but she (not/continue) ……………………… this hobby from next year. 4. If you wash your hands more, you (have)……………………. less chance of catching flu. 5. Getting plenty of rest is very good. It (help)…………………. you to avoid depression. 6. Mr. John (be) ……………………… principal of our school since last year. 7. I (see) ……………………… a car accident on this corner yesterday. 8. We like (come)……………………to school by bus, but we hate (stand)………………………. and (wait) …………………… in the rain. 9. ……………………… (you/ ever/ meet) anyone famous? 10. My mother (come) ……………………… to stay with us next weekend. 11. When he lived in Manchester, he (work) ……………………… in a bank. 12. We (be) ……………………… students for four years. 13. We are planting trees around our school now. Our school (be) ……………………… surrounded by a lot of green trees. 14. What ……………………… your father usually ……………………… (do) in the evenings? 15. My sister likes (cook) ……………………… very much. She can cook many good foods. 16. My father says when he's retired, he (go) …………………… back to his village to do the gardening. 17. They want (buy) ……………………… some meat for dinner. 18. Linh is my best friend. We (know) ……………………… each other for 5 years. 19. I enjoy (fish) ……………………… because it is relaxing. 20. How about (go) ……………………… to the movie theatre?

1 lượt xem
1 đáp án
5 giờ trước