Planets of the Solar system
The collection of the planets of the Solar system is a set because there are exactly eight planets in our Solar system - Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune. Hence, this collection has well defined and distinct elements.
It may be noted that, in the late 1990s, Pluto was excluded from the list of planets as it was considered only as a dwarf planet.
Introduction to Sets
Definition: A collection of well defined and distinct objects is called a set.
Some examples of sets are: the collection of letters of the English alphabet; the collection of official cities in United States; the collection of all countries in the European Union.
Sets are usually denoted by uppercase letters i.e, in capitals.
Each object belonging to the set is called its element or member.
For example, in the set of letters of the English alphabet, each of the letters a, b, c, d, e, ... , x, y, z will be known as its element or member.
Elements are usually denoted by lowercase letters.
The symbol ∈ (epsilon), a Greek Alphabet is used to indicate that an object is an element of a set or belongs to a set.
For example, if the number ‘1’ is an element of the set P, it will be written in symbols as: 1 P.
The symbol ∉ (crossed epsilon) is used to indicate that an object is not an element of a set or does not belong to a set.
Ex: If the number ‘2’ is not an element of the set P, it will be written in symbols as: 2 P.