F6:  -(-a)=a

"-(-a)=a  for any  a  [in]  R.  For both  -(-a)  and  a  are solutions of the equation  x+(-a)=0,  hence equal, by F3."  (see ita-2-1-13-pg18)

ita


Converted [ in part ] by Mathematica      December 4, 2007