Hi Cyrille,

On Fri, Jul 16, 2010 at 5:42 PM, sweedler <[email protected]> wrote:
>  I don't understand where is my mistake.

You didn't make any mistakes. It's a bug in the version of Sage you
were using. The bug has been fixed in the latest version of Sage:

sage: version()
'Sage Version 4.4.4, Release Date: 2010-06-23'
sage: P = (1 + I)*x
sage: Q = -P; Q
(-I - 1)*x
sage: P + Q
0

-- 
Regards
Minh Van Nguyen

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