# twonew - Chapter Two Counting Techniques 2.1 Basic Counting...

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Chapter Two. Counting Techniques 2.1. Basic Counting Rules (or Principles): The Sum Rule: Suppose there are two approaches to completing a task. The first approach produces m results, and the second approach produces n results, where all these results are distinct from one another. Thus, there are a total of m + n results for completing the task (based on the two approaches). A set-theoretic interpretation of the sum rule using Venn diagrams: 2-1, © Dr. S. Lang m results n results Total: m + n results If we let A = the set of m results from approach one, and let B = the set of n results from approach two, then | A | = m , and | B | = n . Thus, | A B | = | A | + | B | = m + n , since A B = . Notes: (1) More generally, | A B | = | A | + | B | – | A B|, when all sets are finite. (2) The sum rule is often used in a complementary fashion, i.e., | A | = | A B | – | B |, if A B = . Example: How many positive integers less than 1000 are not divisible by 12? The positive integers less than 1000 (i.e., from 1 through 999) that are a multiple of 12 include 12, 24, …; the total count equals 999/12 = 83 (the integer quotient using the floor notation). Thus, the count of positive integers less than 1000 not divisible by 12 is 999 – 83 = 916.

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Example: How many positive integers less than 1000 are a multiple of 3 or 5 (or both)? Let A = { n | 1 n 999, and 3 | n }, and let B = { n | 1 n 999, and 5 | n }; that is, A is the set of multiples of 3 less than 1000, and B is the set of multiples of 5 less than 1000. Thus, | A| = 999/3 = 333, and | B| = 999/5 = 199. Note that A B = { n | 1 n 999, and 15 | n } because a multiple of 3 and 5 is exactly a multiple of 15. Thus, | A B| = 999/15 = 66. Therefore, by applying the sum rule, | A B | = | A | + | B | – | A B| = 333 + 199 – 66 = 466. The Product Rule: Suppose it takes two steps to complete a task. There are m paths (methods, or ways) to complete the first step and, for each of these paths, there are n paths to complete the second step. Thus, there are a total of mn paths to complete the two steps (in that order). Path 1 Path m Path 1 Path n Path n Path 1 Total: mn paths Notes: (1) The product rule can be generalized to situations involving more than 2 steps. (2) The product rule applies to situations in which the ordering of the steps is crucial, i.e, a result must follow the order of step 1, then step 2, etc. Step 1 Step 2 2-2, © Dr. S. Lang A graphical representation of the product rule (see figure):
Example: How many 3-digit decimal integers are there? (That is, count the integers from 100, the smallest 3-digit integer, through 999, the largest 3-digit integer.) We can include (or count) all such integers by performing 3 steps: select the first (leftmost) digit, select the middle digit, then select the last (rightmost) digit. There are 9 choices (1 through 9) for the first digit, and for each of these, there are 10 choices (0 through 9) for the middle digit, and

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twonew - Chapter Two Counting Techniques 2.1 Basic Counting...

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