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Number System Complete Revision: Classification, Divisibility, BODMAS for Competitive Exams

Classification of Numbers: The Foundation

Real vs. Imaginary Numbers

  • Real numbers: All numbers from -∞ to +∞ (e.g., -1, 0, 1, 2, 3...)
  • Imaginary numbers: Numbers with a negative under the square root (√-1, √-2...), represented by 'i'.

Important Note: Imaginary numbers don't exist on the real number line.

For a comprehensive guide on real numbers and their classification, check out Understanding and Classifying Real Numbers: A Comprehensive Guide.

Types of Real Numbers: A Quick Breakdown

1. Natural Numbers (N)

  • Definition: Counting numbers given by nature.
  • Set: 1, 2, 3, 4, 5... (starts from 1)
  • Key Rules:
    • Zero is not a natural number.
    • No negative number is a natural number.
    • All natural numbers are positive.

To dive deeper into natural numbers, primes, and integers, explore Mathematics for Data Science: Natural Numbers, Integers, and Primes.

2. Whole Numbers (W)

  • Definition: Natural numbers including zero.
  • Set: 0, 1, 2, 3, 4...

3. Integers (Z)

  • Definition: Includes all positive numbers, negative numbers, and zero.
  • Categories:
    • Positive integers: ...+1, +2, +3...
    • Negative integers: ...-1, -2, -3...
    • Zero: Neutral, neither positive nor negative.

Spotting Numbers: Prime, Composite, Even, Odd, and Perfect

Even and Odd Numbers

  • Even: Set of natural numbers divisible by 2 (form: 2n).
  • Odd: Set of natural numbers not exactly divisible by 2.

Prime Numbers

  • Definition: Numbers divisible only by 1 and themselves.
  • Smallest prime number: 2 (also the only even prime number).
  • Smallest odd prime number: 3.
  • Formula for primes > 5: 6k ± 1 (e.g., for k=7: 7·6 = 42; 42+1=43, 42-1=41, both prime).

Composite Numbers

  • Definition: Numbers divisible by a number other than 1 and itself (more than 2 factors).
  • The number 1: Neither prime nor composite.

Co-Prime Numbers

  • Definition: A pair of numbers with no common divisor other than 1 (HCF = 1).
  • Example: (9, 25), neither number is prime, but together they are co-prime.
  • Rule: Any two consecutive numbers are always co-prime.

Perfect Numbers

  • Definition: A number where the sum of all its divisors (excluding the number itself) equals the number.
  • Example: 6 is perfect because its divisors (1, 2, 3) sum to 6.
  • Counterexample: 3 (only divisor is 1) and 8 (1+2+4=7 ≠ 8) are not perfect.

Smallest & Largest n-Digit Numbers

  • Smallest n-digit number: 10^(n-1)
  • Largest n-digit number: (10^n - 1), which equals n times the digit 9.

Important Summation Formulas

  • Sum of first n natural numbers: n(n+1)/2
  • Sum of squares of first n natural numbers: n(n+1)(2n+1)/6
  • Sum of cubes of first n natural numbers: [n(n+1)/2]²

Rational vs. Irrational Numbers

  • Rational numbers: Can be written in p/q form (q ≠ 0). Includes terminating and repeating decimals.
  • Irrational numbers: Cannot be written in p/q form. Examples: √2, π.
  • Critical distinction:
    • π (pi) is irrational.
    • 22/7 is rational (it's a fraction, not the true value of π).

BODMAS: The Order of Operations

  • Bracket (solved in order: vinculum, , then ( ), then { }, then [ ])
  • Of (multiplication, solved before regular multiplication)
  • Division
  • Multiplication
  • Addition
  • Subtraction

Important: Addition and subtraction are performed left-to-right, not arbitrarily.

Divisibility Rules at a Glance

| Divisor | Rule | |---------|------| | 2 | Last digit is even (divisible by 2). | | 3 | Sum of all digits is divisible by 3. | | 4 | Last two digits are divisible by 4. | | 5 | Last digit is 0 or 5. | | 6 | Divisible by both 2 and 3. | | 8 | Last three digits are divisible by 8. | | 9 | Sum of all digits is divisible by 9. | | 10 | Last digit is 0. |

Solved Example: BODMAS in Action

Question: 5 - [96 ÷ 4 of 3 - (16 - 55 ÷ 5)]

Solution:

  1. Solve inner bracket: 55 ÷ 5 = 11; then 16 - 11 = 5
  2. Now: 5 - [96 ÷ 4 of 3 - 5]
  3. Solve 'of' (multiplication): 4 of 3 = 12
  4. Division: 96 ÷ 12 = 8
  5. Inside bracket: 8 - 5 = 3
  6. Final: 5 - 3 = 2

For a broader revision of similar topics, see Class 10 Maths Half Yearly Marathon: Complete Revision of 8 Chapters.


Key Takeaway: This class is designed to be a definitive revision, every theory point maps directly to exam questions. Practice the BODMAS and rational/irrational conversion problems thoroughly to secure marks in the number system section.

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