Question

which of following is irrational
1) root4/9
2) root 12/3
3) root81

Answer 1

second one is irrational

Answer 2

root4/9 = (2/3)^2 or 2/3 whole square.

root 12/3 = (2√3/√3) whole square.

root 81 = 9square

so second one is irrational