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what is the rule in adding rational numbers
The rule in this case is that there are no numbers in any position.
If it were not true, it would not have become the rule!
Opposite numbers rule
Since a given sequence of numbers can be designed to follow any rule, you have to use a system of trial and error to see if you can discover the rule. Sometimes the rule is obvious, sometimes it is extremely complicated. Try to invent a rule which would produce the sequence that you observe.