It means that the boundaries of the set are not included in the set. For example, consider the set of numbers that are bigger than 1 and smaller than 2. The set is bounded by 1 and 2 but neither of these belong to the set.
Irrational numbers.
2 does belong to the set of imaginary numbers. Any real number is also imaginary. Imaginary numbers are the set of all numbers that can be expressed as a +b*i where "i" is the square root of negative one and "a" and "b" are both real numbers.
The Rationals, the set {1, 3 , 5.86, sqrt(59), -2/3, pi2}, the reals numbers, numbers between 5 and 6, etc.
mixed numbers
The set of numbers which 3 does not belong is the set of even numbers.
10 belongs to the set "natural numbers", but it can also belong to whole numbers, and rational numbers
It means that the boundaries of the set are not included in the set. For example, consider the set of numbers that are bigger than 1 and smaller than 2. The set is bounded by 1 and 2 but neither of these belong to the set.
Counting numbers
Irrational numbers.
A set is just a way of describing numbers, and numbers can be described in more than one way. If set A is (for example) all positive prime numbers, and set B is all numbers between 0 and 10, then there are some numbers (2, 3, 5, and 7) that could belong to both sets.
The set of even numbers
2 does belong to the set of imaginary numbers. Any real number is also imaginary. Imaginary numbers are the set of all numbers that can be expressed as a +b*i where "i" is the square root of negative one and "a" and "b" are both real numbers.
It belongs to the set of prime numbers
rational and prime numbers
Rational and Real numbers
mixed numbers