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fp number is a rational number, why cannot be all numbers?
did you mean to say, "if a number is a rational number, why can't all numbers be?"
a rational number is an integer ie 3 , 11 - 8 or a number that can be written as a fraction a/b. Any other number is irrational eg sqrt 2 - the decimal fom does not end its infinitely long . Other examples of irrational numbers are pi and e.
note not all infinitely recurring numbers are irrational eg 0.3333..... is 1/3 so it is rational.