CLARIFYING ARITHMETIC THROUGH ALGEBRA

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  • CLARIFYING ARITHMETICTHROUGH ALGEBRA

    DAVID X. GORDON1452 Fifty-fifth St., Brooklyn, New York

    For the great majority of students one of the most desirableoutcomes of a course in algebra would be increased comprehen-sion of the fundamental principles of arithmetic. Practicallyall students leaving the elementary schools are ignorant or un-aware of the principles which continually recur and validatetheir mechanical manipulations. If insight into the operationsand nature of arithmetic is to be attained and if fullest use offamiliar arithmetic techniques as background material for thestudy of algebra is to be made, continual stress on underlyingprinciples common to arithmetic and algebra is essential.

    It is the purpose of this article to present situations in thealgebra course of study where this mutual interplay betweenthe two subjects is especially prominent.One of the major obstacles in exhibiting algebra as a generali-

    zation of arithmetic is the loss of identity suffered by the origi-nal numbers in arithmetic operations, thereby concealing theessential similarity between them and their algebraic counter-parts. It is important that this concept be clearly presented atthe outset of a course in algebra and kept before the studentthroughout. Thereby, the student realizes readily that a bi-nomial in algebra may be considered a monomial whose com-ponent parts have not been arithmetically merged and thatevery monomial may be converted into binomial and otherforms.

    BINOMIAL MONOMIALComponent terms not merged Component terms merged

    a-\-b a-\-b (merging postponed)44-3 79-2 7444 41

    Realization that a single quantity such as 4^ may be thoughtof as the sum of two separate terms facilitates recognition of thesimilarity between algebraic situations of the form a(b+c) andarithmetic situations of the form 2X4^. This parallel presenta-tion of the distributive law of multiplication in both subjectsserves as a reminder to pupils that each term of a mixed fraction

    286

  • CLARIFYING ARITHMETIC 287

    must be operated upon by the multiplier. While impressing uponstudents the necessity of recognizing situations subject to thedistributive law, the teacher may wish to use analogous materialin grammar frequently misinterpreted by casual readers. Forinstance, in the sentence "10 years ago, Johns age was twiceHarrys," the adjectival clause "10 years ago" must be "distrib-uted over" the age of each boy.

    MULTIPLICATIONa (b-}-c)2XU2 (4+3)2 couples

    DISTRIBUTIONa b-\-a

    -c

    2X44-2X12X4+2X32 boys and 2 girls

    Extension of the above symbolism to the form (a-{-b)(c+d)is particularly significant, for here the pupils arithmetic showsnoticeable weakness. Students who are asked to multiply 2^ X2^tend to give 5 as an answer with a scattering of 6s and 6^s andand occasional correct response. Identifying this problem as theproduct of two binomials serves to focus attention on the opera-tions indicated by the symbolism and assists considerably inelimination of error.MULTIPLICATION DISTRIBUTION

    (a+b)(c+d) (a+b)c+(a-{-b)d ac+bc+ad+bd(44-3)(54-2) (4+3)5+(4+3)2 4-54-3-54-4-24-3-22^X2^ 2^X24-2^X1 2-24-244-2^4-^

    Vertical forms of multiplication should be employed as wellas horizontal to emphasize clearly the operations used and theirindependence of any particular form. In the case of squaring abinomial, pupils should be made cognizant of the presence ofeach of the four steps of the process even though two of theresulting terms are combined because they are alike.

    (d4-&)2 aa+ab+ba+bb a^+lab+b2Students will appreciate the concepts mentioned above much

    more when they see them applied in arithmetic computation tofacilitate labor. For instance;

    17X 9 17(10 -1) 170-17 15313X99 13(100-1) 1300-13 1287

    The principle of adding like terms merits special considera-tion. If like parts are considered common units and their coeffi-cients are thought of as the number of units, similar terms are

  • 288 SCHOOL SCIENCE AND MATHEMATICS

    added by totaling the number of common units. In this form, theprinciple has wide application in both algebra and arithmetic.LIKE TERMS COMMON UNIT NO. IN EACH TOTAL NO. UNITS

    2 apples+3 apples apple 2, 3 5 apples2 $5 bills+3 $5 bills $5 bill 2, 3 5 $5 bills

    23 14- 2, 3 5 sevenths

    2x-{-3x x 2, 3 5x

    In the case of fractions, the unit fractional part should be pre-sented concretely as a definite entity and the close relation be-tween the word denominator and denomination (or kind), nu-merator and enumerate (or number) pointed out.Another desirable outcome of a course in algebra, not often

    stressed, is release of the pupiPs mind from the impression thatthe form of arithmetic computations is unique. For example,most pupils know only one form for multiplying 32 by 24; whennew arrangements are employed, they are astounded and con-fused. Obviously, they have not learned to differentiate betweenelements of a process which are fundamental and those whichhave organizational value only. This can be corrected by pre-senting such problems as being analogous to binomial productsof the type (10a+6)(10c+d) where the separate products maybe written in any arrangement, provided only that none isomitted. Pupils should be encouraged to devise variations oftheir own and to justify unfamiliar methods on the basis offamiliar principles. Below are listed different arrangements ofthe same problem which are in use today or have been at sometime.

    32 32 32 32^3 23^ 23^ 2396 64 6 73664 96 9 Units 2X3736 736 4 Tens 3X3+2X2 (carry 1)"50 /X5D

    . 6_ Hundreds 3X3736

    Particularly wide variation in the forms used is found in theaddition of fractions. So firmly fixed are the particular methodslearned that pupils find the transition to algebraic forms diffi-cult. Literal representation affords a good opportunity to con-trast essential operations and non-essential patterns, for thereason that numbers do not lose their identity in this form as

  • CLARIFYING ARITHMETIC 289

    they do arithmetically. Novel short methods of adding fractionssuch as the one following will be appreciated all the more be-cause they are understood.

    ALGEBRAIC ARITHMETICa_ c _ ad+bc 2_ 1 _2X2+1X3_ 7T ~d~ bd ~31~3X2~~6

    Analysis of this sort will enable capable students to devise shortcuts of their own. Teachers who do not encourage students toanalyze the methods used in short cuts, puzzles and tricks ofmagic are depriving them in part of awareness of the power ofthe algebraic methods and of the great satisfactions afforded byits knowledge. Material for this purpose can be found in any ofthe many books on mathematical amusements. One such illus-tration is the well known method for squaring a two digit num-ber having the units digit 5. Here, the tens digit is multipliedby the next higher integer and 25 is suffixed.

    (lOa+5)2 100a2+100o+25 100a(o+1)4-25Other number amusements which lend themselves readily toalgebraic clarification are those of the type "Pick a number,double it, add five, etc." Still another interesting device is usedto determine the relative magnitude of two proper fractions.Here, the cross products indicated below reveal which fractionis larger.

    a > c^

    ad^cb >- according as ~ according as ad-cob a . bd doj