## Saturday, December 26, 2015

### [S1_Expository] Recurring Decimals and Rational Numbers

Problem

Introduction
A student asked the above question on Facebook.  This article explains recurring decimals, which is part of the topic on real numbers in the Singapore Secondary 1 Mathematics syllabus.
By the way, I do not believe that Asians are inherently better at mathematics.   A few students are working over Christmas to prepare for the next years’ work.  Here we go!

Notation
Personally, I prefer the horizontal bar notation which is what I learned during my time as a student, but nowadays in Singapore schools, we tend to use the dot notation.  There is no right or wrong about this, but it is just a matter of convention.  When in Rome, do as Romans do.

Coversion to a fraction

Concluding Remarks

A rational number is a number that can be expressed as a ratio or  fraction  p/q  where  p  and  q  are integers with  q ¹ 0.  The fraction can be proper or improper.  Since any recurring decimal can be converted to a fraction,
 every (infinitely) recurring decimal is a rational number.

Now,
 every finitely terminating decimal is also a rational number.
For example, 0.171 = 171/1000.  The numbers that cannot be converted to fractions are called irrational numbers.  How do these numbers look like?
 The irrational numbers are exactly the numbers with non-terminating (infinite) and non-recurring decimal expansions.
Some examples of irrational numbers are
p = 3.141 592 653 589 793 238 462 643 383 279 502 884 197 ¼
e = 2.718 281 828 459 045 235 360 287 471 352 662 497 757 ¼
Ö2 = 1.414 213 562 373 095 048 801 688 724 209 698 078 570 ¼
The decimal digits of irrational numbers never end, but they do not have any repeating pattern.  There are actually much “more” irrational numbers than rational numbers, but this is a fact that is technically profound, way beyond the secondary syllabus.  The interested reader can refer to this article.

Suitable Levels
Lower Secondary Mathematics (Sec 1 ~ Grade 7)
* revision for GCE ‘O’ Level “Elementary” Mathematics (revision)
* other syllabuses that involve recurring decimals
* any independent learner who is interested