For the time being, the proof in the textbook that √2 is not rational number can be put aside, while the counterproposition of the proof shall be figured out at first. It’s not clear whether √2=p/q (p and q are both integers), √2=p/q (p and q are relative primes) or both are the counterpropositions. Even the problem whether there is truth or falsehood in the counterproposition √2=p/q (p and q are relative primes) given in the textbook, has not been considered before. If there is no truth or falsehood, how to apply reductio ad absurdum?