Một lò xo có đầu trên gắn với trần nhà, đầu dưới gắn vật nặng. Trong quá trình vật nặng dao động điều hòa theo phương thẳng đứng, đại lượng không biến thiên điều hòa là đại lượng nào?

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27. My teacher (tell)__________________Hoa about Math. 169. 28. There (be)____________________ animals in the circus. 170. 29. _______________he (watch)______________TV at 7.00 every morning? 171. 30. What _____________she (do) _________________at 7.00 am? 172. 31. How old _________she (be)? 173. 32. How ___________she (be)? 174. 33. My children (Go)________________to school by bike. 175. 34. We (go)_______________to supermarket to buy some food. 176. 35. Mr. Hien (go)________________on business to Hanoi every month. 177. 36. Ha (like)______________coffee very much, but I (not like)______________it. 178. 37. She (like)________________Tea, but she (not like)____________________coffee. 179. 38. I (love)_______________ cats, but I (not love)__________________dogs. 180. 39. Everyday ,I (go)______________to school on foot, but today I (go)________________to school by bike. 181. 40. Who you _________________(wait) for Nam? 182. - No, I _______________________(wait) for Mr. Hai. 183. 41. My sister (get)______________ dressed and (brush)_______________her teeth herself at 6.30 everyday. 184. 42. Mrs. Smith (not live)____________in downtown. She (rent)___________in an apartment in the suburb. 185. 43. How _________your children (go)_________________to school everyday? 186. 44. It's 9 o'clock in the morning. Lien (be)________in her room. She (listen) _______________to music. 187. 45. We_______________________(play) soccer in the yard now. 188. 46. My father (go)___________ to work by bike. Sometimes he(walk)______________. 189. 47. _________You (live)________near a market? _ It (be)____________noisy?