Một nguyên tố X có 2 đồng vị với tỉ lệ nguyên tử là . Hạt nhân nguyên tử X có 35 proton. Trong nguyên tử của đồng vị thứ nhất có 44 nơtron. Số nơtron trong nguyên tử đồng vị thứ hai nhiều hơn trong đồng vị thứ nhất là 2 hạt. Tính nguyên tử khối trung bình của X.

tìm x và yyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyy

Ex2: Choose the best answer 1. Every day I spend two hours ___________ English. A. practise B to practise C. practising D. practised 2. The skiers would rather __________ through the mountains than go by bus. A.to travel by trainB. travel by train C. traveled by train D. traveling by train 3. He likes ___ part in sports, so he joins the football team of the school. A. to take B. to taking C. takes D. took 4. I don't remember___________ the front door when I left home. A. to lock B. lock C. locked D. locking 5. Children enjoy ___________ to ghost stories on Halloween night. A. telling and listening B. tell and listen C. to tell and listen D. to tell and to listen 6. They decided ___________ to Japan for their summer holiday. A. going B. to go C. go D. to going 7. His parents think it's time for him___________ married. A. gets B. get C. to get D. got 8. Peter's father ordered __________ not to stay out late again. A. him B. to him C. he D. for him 9. Smoking is bad for your heath. You had better_________ it up. A. to give B. give C. giving D. to be giving10. I hope you don't mind __________ so late at night. A. telephoning B. to telephone C. telephone D.to have telephoned 11. It took three and a half hours _________ to Singapore. A. to fly B. flying C. to be flown D. fly 12. She refused ___________ them. A. to help B. help C. having helped D. helping 13.My parents can't _________ seeing me at home all day. A. stop B. stand C. start D. hate 14. _________is a popular sport in Europe. A. To ski B. Skiing C. Ski D. To skiing 15. There are many ways of _______ ourselves in a big city. A. enjoy B. enjoying C. enjoyment D. to enjoy 16. I never feel like _______ early on Monday morning. A. getting up B. get up C. gets up D. to get up 17. It is not easy ________ a high-paid job. A. find B. finding C. finds D. to find 18. Would you like the doctor _________ ? A. coming B. to come C. come D. comes 19. I expected them _______me a job. A. offer B. offering C.to be offered D.to offer 20. My husband wants me _______ this letter before afternoon. A. to post B. posting C. post D. posts 21. We all enjoy _________English. A. learn B. to learn C. learning D. learned 22. She doesn't like going _______ in the sea in summer. A. to swim B. to be swimming C. swimming D. swim 23. I am afraid of _______by the strong wind. A. taking away B. take away C. being taken away D. being taking away 24. Your hair is too long now. It needs _________ . A. to cut B. cutting C. being cut D. to be cutting 25. Human beings must stop ______the environment. A. to pollute B. polluted C. to polluted D. polluting 26. We walked for ten kilometers and then we stopped _______ a rest. A. to have B. have C. having D. has 27. It is raining hard. We had better _______ at home. A. staying B to stay C. stay D. to be staying 28. If the earth stops ______ around the sun, there will be no light on earth. A. move B. to move C. moved D. moving 29. She doesn't mind _________ hard to reach her career goals. A. work B. to work C. working D. being worked 30. What should I tell the mechanic? The oil needs ___________. A. changing B. to change C. being changed D. to changing 31. The children enjoy ___________ to the countryside on weekends. A. taking B. to take C. being taken D. to be taken CÁC BẠN GIẢI THK CHI TIẾT GIÚP MK NHA

Các bạn giúp mình với mình đang cần gấp ạ, cảm ơn người đã giúp mình rất nhiều.

Bài 5. Cho nửa đường tròn (O) đường kính AB. Trên AB lấy hai điểm M và N đối xứng nhau qua O. Từ M và N lần lượt kẻ hai đường thẳng song song với nhau cắt (O) tại H và K. Chứng minh tứ giác MNKH là hình thang vuông

V. Complete each of the following sentences in such a way that it means the same as the sentence printed before it (1pt) 1. What does your father do? =>What is …………………………………………………………………. 2. What is the height of this building? => How ……………………………………………………………………………. 3. Minh is the most intelligent student in my class. => No one ………………………………………………………………………. 4. How much does a box of chocolate cost? => What …………………………………………………………………………. 5. Mrs. Tuyet is Trang’s mother’s mother. => Trang …………………………………………………………………………

Cho hình chữ nhật ABCD. Kẻ BH vuông góc với AC. Gọi M, K lần lượt là trung điểm của HC và AD. Chứng minh rằng BM vuông góc với KM.

bạn nào giải giúp mình câu 15 vs mình đang cần trong tối nay, thanks.

tìm và sửa lỗi sai 1.She threw the bag OF potato chips TO James and he CATCHES it WITH one hand A B C D 2.Old Mr.macgill HAD thick red hair when he IS young but now he IS completely BALD A B C D 3.He DIDN'T get job BECAUSE he WASN'T ENOUGH EXPERIENCED A B C D

toán 8 bài 24 sgk trang 80(vẽ hình giúp mình ạ)