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Câu 1. Cho mảnh tôn phẳng đã được gắn vào đầu bút thử điện chạm vào mảnh pôliêtilen đã được cọ xát nhiều lần bằng len thì bóng đèn bút thử điện sáng lên khi chạm ngón tay vào đầu bút vì: A. trong bút đã có điện. B. ngón tay chạm vào đầu bút. C. mảnh poliêtilen đã bị nhiễm điện do cọ xát. D. mảnh tôn nhiễm điện.

35. Minh (play) .........................chess with Nam at the moment. 36. They often (visit) ......................... their parents on Saturday. 37..........................she (study) ......................... now? 38. Mi and I always (go) ......................... to the countryside. 39. He (not be) ......................... at home now. 40. Hanh (not read) ......................... book at the moment. 41. Our teacher usually (give) .........................us many exersices. 42. He often (catch) ......................... a train to work. 43. The cat (catch) ......................... mouse. 44. They (plant) .........................trees overthere at the moment. 45. My old friend, Manh (write) ......................... to me twice a month. 46. What she says (be) ......................... true. 47. Where she (buy) ......................... her breakfast every morning? 48. Bi often (not phone) ......................... to me. 49. Mrs.Green always (take) ......................... a bus to work. But now she (drive) .........................to work. 50. Look! The ball (fall) .........................down.

.,m.................................................

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I. The simple past or the past continuous tense 1.When I arrived the lecture had already started and the professor (write) ……………… on the overhead projector. 2.I (make) ……………… a cake when the light went out. I had to finish it in the dark. 3.I didn't want to meet Paul so when he entered the room I (leave). ………………. 4.He (watch) ……………… TV when the phone rang. Very unwillingly he (turn) ……………… down the sound and (go) ……………… to answer it. 5.He was very polite. Whenever his wife entered the room he (stand) ……………… up. 6.He always (wear) ……………… a raincoat and (carry) …………… an umbrella when he walked to the office. 7.He suddenly (realize) ……………… that he (travel) ……………… in the wrong direction. 8.When I (look) ……………… for my passport I (find) ……………… this old photograph. 9.You looked very busy when I (see) ……………… you last night. What (you/ do)? ………………. 10.He (clean) …..………… his gun when it accidentally (go) …went…………… off and (kill) ……killed………… him. II. The present perfect or the past simple 1.This is my house. ~ How long (you/ live) here ………………? ~I (live) ……………… here since 1970. 2.He (live) ……………… in London for two years and then (go) ……………… to Edinburgh. 3.Shakespeare (write) ……………… a lot of plays. 4.I (not see) ……………… him for three years. I wonder where he is. 5.When ……………… (he/ arrive)? ~ He (arrive) ……………… at 2.00. 6.I (read) ……………… his books when I was at school. I (enjoy) ……………… them very much. 7.I never (drink) ……………… whisky. ~Well, have some now. 8.The clock is slow. ~ It isn't slow, it (stop) ………………. 9.He (go) ……………… out ten minutes ago. 10.……………… (You/ have) breakfast yet? ~ Yes, I ……………… (have) it at 8.00. 11.……………… (You/ see) the moon last night? 12.The play …………………………… (just/ begin). You are a little late. 13.He (break) ……………… his leg in a skiing accident last year. 14.Mr. Pound is the bank manager. He (be) ……………… here for five years. 15.Where is Tom? - I (not see) ……………… him today, but he (tell) ……………… Mary that he'd be in for dinner.

Biết rằng khi nhiệt phân hoàn toàn magie cacbonat (rắn) thì tạo ra magie oxit (MgO) (rắn) và khí cacbonic (CO2). Người ta đem nhiệt phân 80 gam một mẫu magie cacbonat (MgCO3) đến khối lượng không đổi thì thấy có 30 gam magie oxit và 16,8 lít khí CO2. Tính độ tinh khiết của mẫu magie cacbonat nói trên?

giúp mình với bài này ra một số có mũ

a, x^3+1-x^2-x b, (x+y)^3-x3-y3