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I don't have access to mathics but my guess is that you need to define variables x and y in Sage first and then translate  mathics' variables into those in Sage - like:
x, y = var('x y')
sys._sage_(locals={'x':x, 'y':y})
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                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.