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MATHEMATICAL CONCEPTS AND THE DISTRIBUTIVE PROPERTYWhat concepts must student know and understand for a teacher to explain adequately that the product of two negative number is positive? There are many in the background; 1× a = a, 0×a = 0, a+0 = a, a + ( -a ) =0, a + ( b+ c) =(a + b)+ c , and so forth. However , the goal of this article is not to ask whether high school student can reproduce a complete proof but rather to get a feel for the reasoning that might be appropriate for them, given their knowledge of the structure and consistency present in the number system.The most prominent concept required is the distributive property for multiplication over addition: a( b +c ) = ab + ac. Secondary school student should be familiar with the distributive property. Many have probably been using it for some time, ever since they began doing mental mathematics (arithmetic calculations), such as 8×16 = 8(10+6) = 80+48 = 128.To understand (-1)(-1 )= +1, a student must have a more formol algebraic notion of the distributive property (i.e., an understanding that it holds for all the elements in a number set). However, being able to use the distributive property in arithmetic calculations may not mean understanding it algebraically. Matz (1980) and lins (1992) claim that the move from arithmetic calculation to understanding algebraic manipulation is not direct. On the other hand, Vermeulen, Olivier, and Human (1996) explore middle-grades student awareness of the distributive property and describe a teaching strategy that succeeded in increasing the student' algebraic awareness of this property. Evidence suggests that middle school as well as high school student can and do use the distributive property when solving problems.MY ATTEMPT TO ANSWER WHY (-1)(-1) = +1One way to proceed is to add some other expression to (-1)(-1) so that we can use the distributive property. What would that other expression be? Well,it should be the product of two numbers. Moreover, it should include (-1) so that we can use the distributive property (i.e., the two expressions must have a common factor). A good starting point would be (-1)(-1): (-1)(-1)+(-1)(+1) = (-1)(-1+1) by the distributive property =(-1)(0)so (-1)(-1) + (-1)(+1) = 0 Simplifying (-1)(+1), we get (-1)(-1)-1 = 0 Add 1 to both sides yields (-1)(-1)-1+1 = 0+1, or (-1)(-1) = +1Note that I have simply used axioms that govern the number system and that students are already familiar with. This explanation is straightforward and precise; most important, it is not misleading.DiscussionStudent should have developed an intuitive sense that the number system is a well-defined, consistent structure. At the very heart of the number system lie axioms that must always hold: ( a+b) + c = a+(b+c),a-a =0, and the distributive property, for example. Students' success in future algebra classes and postsecondary field and ring theory classes rests on their developing this understanding of number systems. Kiern (1988, 1992) and Booth ( 1988) claim that the difficulty some students experience with algebraic structure is directly linked to previous difficulty with number systems. Accordingly, Kieran and Booth assert that students' previous experience with number systems, their structure and consistency, has an effect on the students' future success with algebra. Some researchers, such as Matz (1980) and Lins (1992), argue that the difficulties students experience with algebra are related not to the numerical context but to the transition from the numerical context to the algebraic context. The difficulty of the transition may hinder their success, but student still need to transfer and apply a solid knowledge of the number system to succeed in an algebraic context.In light of all these discussions, I belive, like Wu, that students need to know that "the product of two negative integers equaling a positive integer is direct consequence of the fact that the distributive property is true for number systems " (2005,p.3) and should not just be asked to "believe for now" or be given some pseudoreasoning. Morever we must consider the ramifications that the presentation such pseudoreasoning may have on our students, future success.BIBLIOGRAPHYBooth,Lesley R. "Children's Difficulties in Beginning Algebra." In The Ideas of Algebra, K - 12,1988 Yearbook of National Council of Teachers of Mathematics (NCTM), edited by Arthur F. Coxford and Albert P. Shulte, pp. 20-32. Reston, VA:NCTM,1988.Edwards, Henry, and David Penny. Calculus. 4th ed.Englewood Cliffs, NJ: Prentice-Hall, 1994.Keedy, Mervin, Marvin Bittinger, and Lord Smith.Algebra Two. Boston: Addison-Wesley,1982.Kieran, Carolyn. "Two Different Approaches among Algebra Learners." In The Ideas of Algebra, k-12,1988 Yearbook of the National Council of Teachers of Mathematics (NCTM), edited by Arthur F. Coxford and Albert P. Shulte,pp. 91-96. Reston,VA: NCTM,1988."The Learning and Teaching of Algebra." In Handbook of Research on Mathematics Teaching and Learning, edited by Douglas Grouws, pp.390-419. New York: Macmillan,1992. Lins, Romulo. " A Framework for Understanding What Algebraic Thinking Is." PhD diss., Shell Centre for Mathematical Education, 1992. Peterson, John C. "Fourteen Different Strategies for Multiplication of Integers or Why (-1)(-1) = +1." The Arithmetic Teacher 19, no. 5 ( May 1972):396-403. Matz, Marilyn. "Building a Metaphoric Theory of Mathematical Thought." Journal of Mathematical Behaviour 3 (1980):93-166. Wu, Hung-His. "Key Mathematical Ideas in Grades 5-8." Paper presented at the annual meeting of the National Council of Teachers of Mathematics,Anaheim, California, april 6-9, 2005. math. berkeley.
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