let x be ext-real number ; :: thesis: ( x in REAL implies - (x + -infty ) = (- -infty ) + (- x) )
assume A1: x in REAL ; :: thesis: - (x + -infty ) = (- -infty ) + (- x)
x + -infty = -infty by A1, Def3;
hence - (x + -infty ) = (- -infty ) + (- x) by Tx4A, A1, Def3; :: thesis: verum