>>13966685actually
if you can visualize the prime factorization of the numerator and denominator
then visualize the prime factorization of the square of the numerator and the square of the denominator
then apply (a/b)^2 = a^2/b^2
you should see that the difference in the powers of the exponent of 2 in the numerator and denominator must be even
and that isn't true of the fraction 2/1
