Fractions and the Index Laws in Algebra
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The Improving Mathematics Education in Schools (TIMES) Project NUMBER AND ALGEBRA Module 32 FRACTIONS AND THE INDEX LAWS IN ALGEBRA A guide for teachers - Years 8–9 June 2011 YEARS 89 Fractions and the Index Laws in Algebra (Number and algebra : Module 32) For teachers of Primary and Secondary Mathematics 510 Cover design, Layout design and Typesetting by Claire Ho The Improving Mathematics Education in Schools (TIMES) Project 2009‑2011 was funded by the Australian Government Department of Education, Employment and Workplace Relations. The views expressed here are those of the author and do not necessarily represent the views of the Australian Government Department of Education, Employment and Workplace Relations. © The University of Melbourne on behalf of the International Centre of Excellence for Education in Mathematics (ICE‑EM), the education division of the Australian Mathematical Sciences Institute (AMSI), 2010 (except where otherwise indicated). This work is licensed under the Creative Commons Attribution‑ NonCommercial‑NoDerivs 3.0 Unported License. 2011. http://creativecommons.org/licenses/by‑nc‑nd/3.0/ The Improving Mathematics Education in Schools (TIMES) Project NUMBER AND ALGEBRA Module 32 FRACTIONS AND THE INDEX LAWS IN ALGEBRA A guide for teachers - Years 8–9 June 2011 Peter Brown Michael Evans David Hunt Janine McIntosh Bill Pender Jacqui Ramagge YEARS 89 {4} A guide for teachers FRACTIONS AND THE INDEX LAWS IN ALGEBRA ASSUMED KNOWLEDGE The arithmetic of integers and of fractions. • Basic methods of mental arithmetic. • The HCF and LCM in arithmetic. • Algebraic expressions with whole numbers and negatives. • The index laws in arithmetic with nonzero whole numbers indices. • The three index laws involving products in algebra. • Linear equations MOTIVATION The various uses of algebra require systematic skills in manipulating algebraic expressions. This module is the third of four modules that provide a systematic introduction to basic algebraic skills. In the module, Algebraic Expressions, we introduced algebra using only whole numbers and the occasional fraction for the pronumerals. Then in the module Negatives and the Index Laws in Algebra we extended the methods of that module to expressions involving negatives. The present module extends the methods of algebra so that all negative fractions and decimals can also be substituted into algebraic expressions, and can appear as the solutions of algebraic equations. Although earlier modules occasionally used negative fractions, this module provides the first systematic account of them, and begins by presenting the four operations of arithmetic, and powers, in the context of negative fractions and decimals. The resulting number system is called the rational numbers. This system is sufficient for all the normal calculations of every life, because adding, subtracting, multiplying and dividing rational numbers (apart from division by zero), and taking whole number powers of rational numbers, always produces another rational numbers. In particular, scientific calculations often involve manipulation of equations with decimals. The Improving Mathematics Education in Schools (TIMES) Project {5} The module also begins the discussion of algebraic fractions, which routinely cause problems for students. In this module, the discussion is quickly restricted by excluding numerators and denominators with more than one term. These more complicated algebraic fractions are then covered in more detail in the fourth module, Special Expansions and Algebraic Fractions. Once fractions have been introduced into algebra, the two index laws dealing with quotients can be stated in a more systematic form, and then integrated with algebra. This completes the discussion of the index laws in arithmetic and algebra as far as nonzero whole number indices are concerned. CONTENT THE RATIONAL NUMBERS Our arithmetic began with the whole numbers 0, 1, 2, 3, 4,… We then constructed extensions of the whole numbers in two different directions. First, in 2 9 1 the module, Fractions we added the positive fractions, such as 3 and 2 = 42, which can be written as the ratio of a whole number and a non–zero whole number, and we showed how to add, subtract, multiply and divide fractions. This gave a system of numbers consisting of zero and all positive fractions. Within this system, every non–zero number has a reciprocal, and division without remainder is always possible (expect by zero). Only zero has an opposite, however, and a subtraction a – b is only possible when b ≤ a. Secondly, in the module, Integers we started again with the whole numbers and added the opposites of the whole numbers. This gave a system of numbers called the integers, consisting of all whole numbers and their opposites, …, –3, –2, –1, 0, 1, 2, 3,… and we showed how to add, subtract, multiply and divide integers. Within this system of integers, every number has an opposite, and subtraction is always possible. Only 1 and –1 have reciprocals, however, and division without remainder is only possible in situations such as 21 ÷ 7 when the dividend is an integer multiple of the divisor. {6} A guide for teachers Arithmetic needs to be done in a system without such restrictions – we want both subtraction and division to be possible all the time (except for division by zero). To achieve this, we start with the integers and positive fractions that we have already. Then we add the opposites of the positive fractions, so that our number system now contain all numbers such as 2 9 1 –3 and –2 = –42 All these numbers together are called the rational numbers – the word ‘rational’ is the adjective from ‘ratio’. We have actually been using negative fractions in these notes for some time. These remarks are intended to formalise the situation. The usual laws of signs apply to multiplying and dividing in this larger system, so that a fraction with negative integers in the numerator, or in the denominator, or in both, can be written as either a positive or a negative fraction, –2 2 2 –2 2 3 and –3 = –3 and –3 = 3 Because of these identities, it is usual to define a rational number as the ratio of an integer and a non–zero integer: ‘A rational number is a number that can be written as a fraction, where a is an integer and b is a non–zero integer’. Thus positive and negative mixed numerals, terminating decimals, and recurring decimals are rational numbers because they can be written as fractions of integers. For example, 1 46 –23 917 –12 95 = 5 , –23.917 = 1000 , –1.09 = 11 In later modules on topics such as surds, circles, trigonometry and logarithms, we will need to introduce further numbers, called irrational numbers, that cannot be written as fractions. Some examples of such numbers are 2 , π, sin 22°, log2 5. The module, The Real Numbers will discuss the resulting more general system of arithmetic. EXAMPLE a Show that each number below is rational by writing it as a fraction b, where a and b are integers with b ≠ 0. 3 1 a 5 b –7 c 53 d 0.002 e –4.7 f 1.3 SOLUTION 5 3 –3 1 16 a 5 = 1 b –7 = 7 c 53 = 3 2 –47 4 d 0.002 = 1000 e –4.7 = 10 f 1.3 = 3 The Improving Mathematics Education in Schools (TIMES) Project {7} Constructing the rational numbers There are two ways to construct the rational numbers. The first way is to construct the positive fractions first and then the negatives, which is roughly what happened historically. This involves a three–step procedure: 1 First construct the whole numbers 0, 1, 2, 3, 4,… 3 9 2 Then construct the non–negative fractions, such as 4 and 4, as the ratio of two whole numbers, where the second is non–zero. This process involves identifying a fraction 2 such as 6, where the two whole numbers have a common factor, with its simplest 1 5 form 3, and identifying a fraction such as 1 , where the denominator is 1, with the whole number 5. 3 Lastly construct the rational numbers by constructing the opposites of every positive fraction and whole number. The other method is to construct the integers first and then the fractions. The three steps are now: 1 First construct the whole numbers 0, 1, 2, 3, 4,.. 2 Then construct the integers by constructing the opposites …, –4, –3, –2, –1 of all the nonzero whole numbers. 3 3 Lastly, construct the rational numbers by constructing all the fractions such as 4 and 9 4 the ratio of two integers, where the second is non–zero. This process again involves identifying equivalent fractions, and identifying fractions with denominator 1 as integers. These two procedures are equally acceptable mathematically, and their final results are identical in structure. Students learn about rational numbers, however, by working with examples of them in arithmetic and algebra, and there is little to be gained by such explanations. ADDING AND SUBTRACTING NEGATIVE FRACTIONS AND DECIMALS When adding and subtracting fractions and decimals that may be positive or negative, the rules are a combination of the rules for integers and the rules for adding and subtracting positive fractions and decimals: • To subtract a negative number, add its opposite. • Perform addition and subtraction from left to right. • When dealing with mixed numerals, it is usually easier to separate the integer part and the fraction. {8} A guide for teachers The following examples demonstrate such calculations first in arithmetical examples, then in substitutions and equations. EXAMPLE Simplify: 5 3 1 a 8 + –4 – –4 b 32.6 + (–23.6) – (–26.4) SOLUTION 5 3 1 5 6 2 a 8 + –4 – –4 = 8 – 8 + 8 b 32.6 + (–23.6) – (–26.4) = 32.6 – 23.6 + 26.4 1 = 8. = 35.4. EXAMPLE Evaluate –x – y + z when 2 1 3 a x = –203, y = 252 and z = –354.