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Introduction
Introduce addition, subtraction, multiplication, and division of integers.
Show that the only change to these operations is to keep track of the sign of the result.
Illustrate clearly why a * -b = -b * a = -ab and why -a * -b = ab (use properties of addition).
Show that -a = -1 * a. Therefore, a * -b = a * b * -1.
Therefore, -a * -b = -1 * -1 * a * b and -1 * -1 = 1
1 + -1 = 0
a * (1 + -1) = 1*a + -1*a = a + -a = 0
-1 + 1 = 0
-1 * (-1 + 1) = 0
-1 * -1 + -1 * 1 = 0
(-1*-1) + -1 = 0
(-1*-1) = 1
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