1, all positive even numbers greater than 0 and less than 20.

{x|x=2k，0 & ltk & lt 10，k∈z}

2. The set of all natural numbers greater than 0.9 and less than 3.9.

{ 1,2,3}

3. All the people in a group are divided by 3 into 1.

{X|X=3k+ 1，k∈z}

4. All positive factors of a set of 15.

{ 1,3,5, 15}

5. The set of all real numbers whose absolute value is equal to 2.

{-2,2}

The set of the square roots of all 6 and 9.

{-3,3}