第20题: [单项选择]方程x2+y2+z2-2x+4y=0表示的曲面是()。 A. 球面 B. 双曲面 C. 抛物面 D. 锥面 参考答案:A 答案解析:
方程化为(x-1)2+(y+2)2+z2=5,它是以(1,-2,0)为中心,半径为的球面。
第21题: [单项选择]What happened while Mrs. Jones was cleaning the house A. She ran back and prepared everything to wait for her husband. B. She quickly cleaned her house. C. She quickly wore her best clothes. 参考答案:A