(i) A = {x : x² = 4 and 3x = 9}.

x² = 4 ⇒ x = ±

3x = 9 ⇒ x =

The value of ‘x’ is not same in both cases, so this is a set.

(ii) The set of all triangles in a plane having the sum of their three angles less than 180.

This is a set, because the sum of the three angles of a triangle is equal to °.