Chapter 1: Number Systems

Real Numbers & Decimal Expansions

Real Numbers and Decimal Expansions

The Complete Picture

Real Numbers = Rational Numbers + Irrational Numbers

Every point on the number line represents a real number!


Decimal Expansions of Rationals

Type 1: Terminating Decimals

FractionDecimalWhy it terminates
1/20.5Denominator = 2
3/40.75Denominator = 4 = 2²
7/80.875Denominator = 8 = 2³
1/50.2Denominator = 5

Rule: Fraction terminates if denominator has ONLY 2s and 5s as prime factors!


Type 2: Repeating (Recurring) Decimals

FractionDecimalRepeating block
1/30.333...3 repeats
1/70.142857142857...142857 repeats
1/110.090909...09 repeats
1/60.1666...6 repeats

Key Theorem

The decimal expansion of a rational number is either terminating or non-terminating recurring.

The decimal expansion of an irrational number is non-terminating non-recurring.


Example: Predicting 2/7, 3/7, etc. from 1/7

Since 1/7 = 0.142857...

  • 2/7 = 0.285714... (starts from 2 in the pattern)
  • 3/7 = 0.428571... (starts from 4)
  • 4/7 = 0.571428... (starts from 5)
  • 5/7 = 0.714285... (starts from 7)
  • 6/7 = 0.857142... (starts from 8)

All use the SAME digits 142857, just different starting points!

Visualizer
Fraction Addition
1
7
+
2
7

Interactive Visualization