|x + y| =< |x| + |y|, |x - y| >= |x| - |y|, x + y = x.y, x.y/x = y, (x + y) - (x - y), | x| + | y| ≤ | x + y|, ( x + y ).( x - y ), (x + y) + ( x - y), (x+y).(x+y), (x - y)(x + y), (x + y)² - (x - y)², (x + y)³(x - y)³, (x + y) + (x - y) ;