Explanation:
a rational number can be always expressed as fraction of two integers : x/y
if there would be a rational number with its square being 3, then we would have
x²/y² = 3
x² = 3y²
so, the squared rational number would have to be
3y²/y²
therefore, 3y² would have to be a squared number too.
and the basic number would then be sqrt(3)×y, which is not an integer. so, there can't be such a rational number.