Sets
Maharashtra Board · Class 9 · Mathematics
Quick revision notes for Sets — Maharashtra Board Class 9 Mathematics. Key concepts, formulas, and definitions for last-minute revision.
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Key Topics to Revise
Introduction to Sets and Methods of Representation
- A set is a well-defined collection of objects. The objects must be clearly identifiable — no ambiguity allowed.
- Collections like 'happy children' or 'brilliant students' are NOT sets because the terms 'happy' and 'brilliant' are relative and differ from person to person.
- Collections like 'days of a week', 'planets in our solar system', 'first 10 natural numbers' ARE sets because membership is clearly defined.
Types of Sets
- Singleton Set: A set with exactly ONE element. Example: A = {2} (set of even prime numbers).
- Empty Set / Null Set: A set with NO elements. Represented by {} or φ (phi). Example: Set of natural numbers between 2 and 3 = φ.
- Finite Set: A set where the number of elements can be counted (including the empty set). Example: C = {4, 8, 12, 16, 20}.
Equal Sets, Subsets, and Venn Diagrams
- Equal Sets: Two sets A and B are equal (A = B) if they have EXACTLY the same elements — order does not matter.
- If even ONE element is different, the sets are NOT equal (A ≠ B).
- Example: A = {l,i,s,t,e,n} and B = {s,i,l,e,n,t} → A = B (same elements, different order).
Universal Set and Complement of a Set
- Universal Set (U): The larger set that contains all the sets under consideration as subsets. Every set in a problem is a subset of U.
- The Universal set depends on the context of the problem. For example, if we study sets of numbers, U could be the set of all integers.
- In Venn diagrams, the Universal set is represented by a RECTANGLE.
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