Sets
Basic Definitions
Set: Well-defined collection of objects. Element: Member of a set. a ∈ A.
Types of Sets
| Type | Definition |
|---|---|
| Empty (Null) | No elements. ∅ or {} |
| Finite | Countable number of elements |
| Infinite | Unlimited elements |
| Equal | Exactly the SAME elements |
Subsets: A ⊆ B if every element of A is in B. Proper subset: A ⊂ B (A ⊆ B and A ≠ B).
Set Operations
- Union A ∪ B: Elements in A OR B (or both)
- Intersection A ∩ B: Elements in BOTH A and B
- Difference A − B: Elements in A but NOT in B
- Complement A′: Elements NOT in A
Cardinal Number: n(A ∪ B) = n(A) + n(B) − n(A ∩ B)
Venn Diagrams — Draw overlapping circles to visualise set relationships.
Detailed Set Operations
Union of Sets (A ∪ B)
The set of ALL elements that belong to A OR B (or both).
Example 1: If A = {1, 2, 3, 4} and B = {3, 4, 5, 6}, then A ∪ B = {1, 2, 3, 4, 5, 6} ✓ 'List each element only ONCE, even if it appears in both sets.'
Intersection of Sets (A ∩ B)
The set of elements common to BOTH A and B.
Example 2: Using the same A and B: A ∩ B = {3, 4} ✓
Difference of Sets (A − B)
The set of elements in A but NOT in B.
Example 3: A − B = {1, 2} (elements in A that are NOT in B) B − A = {5, 6} (elements in B that are NOT in A)
IMPORTANT: A − B ≠ B − A (set difference is NOT commutative)
Complement of a Set (A′ or Aᶜ)
Given a Universal Set U, A′ = {x ∈ U : x ∉ A}
Example 4: If U = {1, 2, 3, 4, 5, 6, 7, 8} and A = {2, 4, 6, 8}, then A′ = {1, 3, 5, 7} ✓
Venn Diagrams — Visualising Set Operations
How to Draw a Venn Diagram
- Draw a RECTANGLE to represent the UNIVERSAL SET (U).
- Draw CIRCLES inside for each set.
- Shade the RELEVANT region for the operation.
Venn Diagrams for Common Operations
A ∪ B: Shade ALL regions inside both circles (including overlap). A ∩ B: Shade ONLY the OVERLAPPING region between the two circles. A − B: Shade the part of circle A that does NOT overlap with circle B. A′: Shade everything OUTSIDE circle A (but inside U).
'For THREE sets, draw three overlapping circles. The centre belongs to ALL three sets.'
Example with Three Sets
If U = {1, 2, 3, 4, 5, 6, 7, 8, 9, 10}, A = {1, 2, 3, 4, 5}, B = {3, 4, 5, 6, 7}, C = {5, 7, 8, 9}:
n(A) = 5, n(B) = 5, n(C) = 4 n(A ∩ B) = 3, n(B ∩ C) = 2, n(A ∩ C) = 1, n(A ∩ B ∩ C) = 1
Cardinality of Sets
The CARDINAL NUMBER n(A) is the NUMBER of elements in set A.
Cardinality Formulas
| Formula | Meaning |
|---|---|
| n(A ∪ B) = n(A) + n(B) − n(A ∩ B) | Two sets — SUBTRACT overlap once |
| n(U) = n(A) + n(A′) | Universal set = set + its complement |
| n(A ∪ B ∪ C) = n(A) + n(B) + n(C) − n(A∩B) − n(B∩C) − n(A∩C) + n(A∩B∩C) | Three sets — inclusion-exclusion principle |
'For two sets, subtract the intersection ONCE. For three sets, subtract PAIRWISE intersections, then ADD BACK the triple intersection.'
Worked Example — Two Sets
In a class of 60 students, 35 like Mathematics, 28 like Science, and 15 like BOTH. Find how many like: a) At least one subject b) Neither subject c) Only Mathematics d) Only Science
n(M) = 35, n(S) = 28, n(M ∩ S) = 15
a) n(M ∪ S) = n(M) + n(S) − n(M ∩ S) = 35 + 28 − 15 = 48 ✓ b) n(Neither) = n(U) − n(M ∪ S) = 60 − 48 = 12 ✓ c) Only M = n(M) − n(M ∩ S) = 35 − 15 = 20 ✓ d) Only S = n(S) − n(M ∩ S) = 28 − 15 = 13 ✓
'Always draw the Venn diagram first. It helps you SEE the numbers before you calculate.'
Worked Example — Three Sets
In a survey of 100 families, 55 read Times of India, 40 read Hindu, 35 read Deccan Chronicle. 20 read TOI and Hindu, 15 read Hindu and DC, 10 read TOI and DC, and 5 read ALL three. Find: a) Families reading at least one newspaper b) Families reading exactly one newspaper
Let T = TOI, H = Hindu, D = Deccan Chronicle
n(T ∪ H ∪ D) = n(T) + n(H) + n(D) − n(T∩H) − n(H∩D) − n(T∩D) + n(T∩H∩D) = 55 + 40 + 35 − 20 − 15 − 10 + 5 = 90 ✓
90 families read at least one newspaper.
For exactly one: Only T = n(T) − n(T∩H) − n(T∩D) + n(T∩H∩D) = 55 − 20 − 10 + 5 = 30 Only H = n(H) − n(T∩H) − n(H∩D) + n(T∩H∩D) = 40 − 20 − 15 + 5 = 10 Only D = n(D) − n(H∩D) − n(T∩D) + n(T∩H∩D) = 35 − 15 − 10 + 5 = 15 Exactly one = 30 + 10 + 15 = 55 ✓
De Morgan's Laws
Law 1: (A ∪ B)′ = A′ ∩ B′ 'The complement of a UNION is the INTERSECTION of the complements.' Law 2: (A ∩ B)′ = A′ ∪ B′ 'The complement of an INTERSECTION is the UNION of the complements.'
Verification with Example
Let U = {1, 2, 3, 4, 5, 6, 7, 8}, A = {2, 4, 6}, B = {1, 2, 3, 4}
A ∪ B = {1, 2, 3, 4, 6} → (A ∪ B)′ = {5, 7, 8} A′ = {1, 3, 5, 7, 8}, B′ = {5, 6, 7, 8} → A′ ∩ B′ = {5, 7, 8} ✓ Hence (A ∪ B)′ = A′ ∩ B′ — De Morgan's 1st Law VERIFIED.
A ∩ B = {2, 4} → (A ∩ B)′ = {1, 3, 5, 6, 7, 8} A′ ∪ B′ = {1, 3, 5, 7, 8} ∪ {5, 6, 7, 8} = {1, 3, 5, 6, 7, 8} ✓ Hence (A ∩ B)′ = A′ ∪ B′ — De Morgan's 2nd Law VERIFIED.
Types of Sets — Detailed
| Type | Definition | Example |
|---|---|---|
| Empty (Null) Set | No elements. Symbol: ∅ or {} | { } or ∅ |
| Singleton Set | Exactly ONE element | {5} |
| Finite Set | Countable number of elements | {a, b, c} |
| Infinite Set | Countless elements | ℕ = {1, 2, 3, ...} |
| Equal Sets | Identical elements (order irrelevant) | {1, 2} = {2, 1} |
| Equivalent Sets | SAME number of elements | A = {p, q}, B = {2, 4} → n(A) = n(B) = 2 |
| Disjoint Sets | NO common elements. A ∩ B = ∅ | A = {1, 2}, B = {3, 4} |
| Power Set | Set of ALL subsets. n[P(A)] = 2ⁿ | A = {a, b}, P(A) = {∅, {a}, {b}, {a, b}} |
'Power set includes the EMPTY SET and the SET ITSELF. For a set with n elements, the power set has 2ⁿ elements.'
Subsets — Deep Dive
A ⊆ B means EVERY element of A is in B.
Proper Subset (A ⊂ B): A ⊆ B BUT A ≠ B (at least one element of B is missing in A).
Number of Subsets of a set with n elements:
- Total subsets: 2ⁿ
- Proper subsets: 2ⁿ − 1 (exclude the set itself)
- Non-empty subsets: 2ⁿ − 1 (exclude empty set)
- Non-empty proper subsets: 2ⁿ − 2
Example: For A = {x, y, z}, n = 3 Total subsets = 2³ = 8: ∅, {x}, {y}, {z}, {x,y}, {x,z}, {y,z}, {x,y,z} Proper subsets = 8 − 1 = 7 (exclude {x,y,z}) Non-empty subsets = 8 − 1 = 7 (exclude ∅)
Common Mistakes
- 'Set {0} is empty' — No! {0} has ONE element (0). The empty set is {} or ∅.
- 'Order matters in sets' — NO! {1, 2, 3} = {3, 1, 2}. Sets care about MEMBERSHIP, not ORDER.
- 'A − B = B − A' — FALSE. Set difference is NOT commutative.
- 'n(A ∪ B) = n(A) + n(B)' — Wrong unless A and B are DISJOINT. Always subtract n(A ∩ B).
- '∅ and {∅} are the same' — Different. ∅ has 0 elements. {∅} has 1 element (the empty set).
AP SSC Board Exam Focus
| Topic | Marks | Frequency |
|---|---|---|
| Set operations (∪, ∩, −, ′) | 2-3 | Very High |
| Cardinality problems (2 sets) | 4 | Very High |
| Venn diagram interpretation | 3-4 | High |
| De Morgan's laws | 2 | Moderate |
| Types of sets | 1-2 | Moderate |
| Power set and subsets | 2 | Moderate |
| Cardinality problems (3 sets) | 4 | High |
Self-Test Questions
- If A = {x : x is a prime number less than 15} and B = {x : x is an odd number less than 15}, find A ∪ B, A ∩ B, A − B, and B − A.
- In a school of 500 students, 280 play cricket, 180 play football, and 50 play BOTH. How many play NEITHER sport?
- In a group of 200 people, 120 like tea, 110 like coffee, and 30 like NEITHER. How many like BOTH tea and coffee?
- If U = {1, 2, 3, 4, 5, 6, 7, 8, 9, 10}, A = {2, 4, 6, 8, 10}, B = {1, 3, 5, 7, 9}, verify De Morgan's laws.
- For set A = {a, b, c, d}, write the power set and find the number of proper subsets.
- In a class, 50 students took Hindi, 40 took English, 30 took BOTH. If the class has 80 students, how many took NEITHER?
- If n(A) = 25, n(B) = 30, n(A ∪ B) = 45, find n(A ∩ B).
- Prove that A ∩ (B ∪ C) = (A ∩ B) ∪ (A ∩ C) using set membership.
Answers: 1) A = {2,3,5,7,11,13}, B = {1,3,5,7,9,11,13}, A∪B={1,2,3,5,7,9,11,13}, A∩B={3,5,7,11,13}, A−B={2}, B−A={1,9}, 2) 90, 3) 60, 4) Verified above, 5) 16 subsets, 15 proper, 6) 20, 7) 10, 8) Use membership table
