|   | 1 |  initial version  | 
Sage symbolics handles fractional powers better than numerical ones, so if you replace
U=x^0.7*y^0.3;U
with
U=x^(7/10)*y^(3/10);U
it seems to work much better.
 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.