x [< y <==> (-y) [< (-x)
y >] x <==> (-x) >] (-y)
0 [< x <==> (-x) [< x
x >] 0 <==> x >] (-x)
x [< 0 <==> x [< (-x)
0 >] x <==> (-x) >] x
demostració:
x [< y
(-x)+x+(-y) [< (-x)+y+(-y)
(-y) [< (-x)
x+(-y)+y [< x+(-x)+y
demostració:
(-x) [< 0 & 0 [< x
absurd: Si x < 0 ==>
(-x) [< x & x < 0
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