|   | 1 |  initial version  | 
The problem comes from this line:
x=V[0];y=V[1]
If V is a matrix, then x and y become rows, not symbolic variables. In the next line x^2 triggers the error, since a row squared doesn't make sense. Simply Replace it with 
x=V[0, 0]; y=V[1, 0]
 Copyright Sage, 2010. Some rights reserved under creative commons license. Content on this site is licensed under a Creative Commons Attribution Share Alike 3.0 license.
 
                
                Copyright Sage, 2010. Some rights reserved under creative commons license. Content on this site is licensed under a Creative Commons Attribution Share Alike 3.0 license.