Your demo is valid if the postulates of real numbers are known by the student, since it uses the identity element, hence it is necessary for (-1)(-1) to be 1.
On Tue, Nov 27, 2012 at 8:12 AM, Peter Duveen <email@example.com> wrote: > I have a new student, who was confused about negative numbers, both multiplication and division of the same. I wanted to demonstrate to him that a negative times a negative is a positive, but got flustered because I could not produce the demonstration. I promised to show him the demonstration at our next weekly meeting. > > Later, I sat down and derived the following demonstration: > > Proof that the multiplication of two negative numbers is a positive number: - 1 + 1 = 0 (Definition of -1); -1(-1 + 1) = 0 (0 times any number is 0); -1x-1 + -1x1 = 0 (distributive law of multiplication); -1x-1 + -1 = 0 (1 x any number is that number itself); -1x-1 = 1 (definition of -1; the same quantity added to equal quantities produce equal quantities). > > Main point is, it is not particularly self-evident that a negative number times a negative number yields a positive number. Some will undoubtedly argue that to demonstrate this property rather than to just state it will confuse the student. At the same time, I would argue that to not demonstrate it will confuse the student. > > I'm just throwing this out for discussion, but other properties that ought to be demonstrated are that 1/-a = -(1/a), another property that many may feel does not need to be demonstrated. On the contrary, I believe that not demonstrating it will lead to confusion.
- -- Jim
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