Lots of numbers do.
To begin, all real numbers do.
Multiples of sqrt(-1), aka. imaginary numbers, do.
The Complex Numbers are all numbers which are the sum of a real number and an imaginary number.
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9 belongs in the sets: -Natural number set, positive whole numbers -Integer number set, whole numbers -Rational number set, numbers that are not never ending -Real number set, basic numbers without i and that can be expressed in say amounts of apples -Complex number set, the set that contains both real and unreal numbers
It belongs to any set that contains it: for example, {4.75, -12, pi, sqrt(5), 29}. It belongs to the set of integers which is a proper subset of rational numbers which is a proper subset of real numbers which is a proper subset of complex numbers. So -12 belongs to all the above sets.
The set of complex numbers is the set of numbers which can be described by a + bi, where a and b are real numbers, and i is the imaginary unit sqrt(-1). Since a and b can be any real number (including zero), the set of real numbers is a subset of the set of complex numbers. Also the set of pure imaginary numbers is a subset of complex number set.
Always. The set of imaginary numbers is a subset of complex numbers. Think of complex numbers as a plane (2 dimensional). The real numbers exist on the horizontal axis. The pure imaginary are the vertical axis. All other points on the plane are combinations of real and imaginary. All points on the plane (including imaginary axis and real axis) are complex numbers.
There is no such number. All of these sets go on forever.