Explicație pas cu pas:
Aplicam |x|+|y|>=|x+y|
|x-1|+|x-2|>=|x-1+x-2| => |x-1|+|x-2|>=|2x-3|
Stim ca |x|=|-x| deci putem spune ca |x-3|=|3-x|
|x-1|+|x-2|>=|2x-3| => |x-1|+|x-2|+|3-x|>=|2x-3|+|3-x|
dar |2x-3|+|3-x|>=|2x-3+3-x| => |2x-3|+|3-x|>=|x|
deci |x-1|+|x-2|+|3-x|>=|x| <=> |x-1|+|x-2|+|3-x|>=|x|