指出下列各方程表示哪种曲面:
(1)x2+y2-2z=0; (2)x2=4y; (3)z2-x2-y2=0; (4)4x2+y2-z2=4.
指出下列方程表示的是什么曲面? (1)x2+y2+z2-2x+4y+2z=0; (2)x2+y2+2z2=1; (3)x2+y2+2z2=0; (4)x2+y2-2z2=0; (5)x2+y2-2z2=1; (6)x2-y2-2z2=1; (7)x2+2y2=z; (8)-x2+2y2=z; (9)x2+y2-1=0; (10)x2-y2=1; (11)x2+2y2=1; (12)xyz=0.