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**Number Theory and the Real Number System**

CHAPTER 5 Number Theory and the Real Number System

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**Number Theory: Prime & Composite Numbers**

5.1 Number Theory: Prime & Composite Numbers

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**Objectives Determine divisibility.**

Write the prime factorization of a composite number. Find the greatest common divisor of two numbers. Solve problems using the greatest common divisor. Find the least common multiple of two numbers. Solve problems using the least common multiple.

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**Number Theory and Divisibility**

Number theory is primarily concerned with the properties of numbers used for counting, namely 1, 2, 3, 4, 5, and so on. The set of natural numbers is given by Natural numbers that are multiplied together are called the factors of the resulting product.

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Divisibility If a and b are natural numbers, a is divisible by b if the operation of dividing a by b leaves a remainder of 0. This is the same as saying that b is a divisor of a, or b divides a. This is symbolized by writing b|a. Example: We write 12|24 because 12 divides 24 or 24 divided by 12 leaves a remainder of 0. Thus, 24 is divisible by 12. Example: If we write 13|24, this means 13 divides 24 or 24 divided by 13 leaves a remainder of 0. But this is not true, thus, 13|24.

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Prime Factorization A prime number is a natural number greater than 1 that has only itself and 1 as factors. A composite number is a natural number greater than 1 that is divisible by a number other than itself and 1. The Fundamental Theorem of Arithmetic Every composite number can be expressed as a product of prime numbers in one and only one way. One method used to find the prime factorization of a composite number is called a factor tree.

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**Example 2: Prime Factorization using a Factor Tree**

Example: Find the prime factorization of 700. Solution: Start with any two numbers whose product is 700, such as 7 and 100. Continue factoring the composite number, branching until the end of each branch contains a prime number.

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**Example 2 (continued) Thus, the prime factorization of 700 is**

700 = 7 2 2 5 5 = 7 22 52 Notice, we rewrite the prime factorization using a dot to indicate multiplication, and arranging the factors from least to greatest.

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**Greatest Common Divisor**

To find the greatest common divisor of two or more numbers, Write the prime factorization of each number. Select each prime factor with the smallest exponent that is common to each of the prime factorizations. Form the product of the numbers from step 2. The greatest common divisor is the product of these factors. Pairs of numbers that have 1 as their greatest common divisor are called relatively prime. For example, the greatest common divisor of 5 and 26 is 1. Thus, 5 and 26 are relatively prime.

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**Example 3: Finding the Greatest Common Divisor**

Example: Find the greatest common divisor of 216 and 234. Solution: Step 1. Write the prime factorization of each number.

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**Example 3: (continued) 216 = 23 33 234 = 2 32 13**

216 = 23 = 2 32 13 Step 2. Select each prime factor with the smallest exponent that is common to each of the prime factorizations. Which exponent is appropriate for 2 and 3? We choose the smallest exponent; for 2 we take 21, for 3 we take 32.

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Example 3: (continued) Step 3. Form the product of the numbers from step 2. The greatest common divisor is the product of these factors. Greatest common divisor = 2 32 = 2 9 = 18. Thus, the greatest common factor for 216 and 234 is 18.

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Least Common Multiple The least common multiple of two or more natural numbers is the smallest natural number that is divisible by all of the numbers. To find the least common multiple using prime factorization of two or more numbers: Write the prime factorization of each number. Select every prime factor that occurs, raised to the greatest power to which it occurs, in these factorizations. Form the product of the numbers from step 2. The least common multiple is the product of these factors.

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**Example 5: Finding the Least Common Multiple**

Example: Find the least common multiple of 144 and 300. Solution: Step 1. Write the prime factorization of each number. 144 = 24 32 300 = 22 3 52 Step 2. Select every prime factor that occurs, raised to the greatest power to which it occurs, in these factorizations = 24 32

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Example 5: continued Step 3. Form the product of the numbers from step 2. The least common multiple is the product of these factors. LCM = 24 32 52 = 16 9 25 = 3600 Hence, the LCM of 144 and 300 is Thus, the smallest natural number divisible by 144 and 300 is 3600.

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**Example 6: Solving a Problem Using the Least Common Multiple**

A movie theater runs its films continuously. One movie runs for 80 minutes and a second runs for 120 minutes. Both movies begin at 4:00 P.M. When will the movies begin again at the same time? Solution: Movie 1 4: | 5: | 6: | 8:00 Movie 2 4: | 6: | 8:00

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**Example 6: Solving a Problem Using the Least Common Multiple**

A movie theater runs its films continuously. One movie runs for 80 minutes and a second runs for 120 minutes. Both movies begin at 4:00 P.M. When will the movies begin again at the same time? Solution: We are asked to find when the movies will begin again at the same time again. What is the time duration (minutes) into which 80 and 120 will divide evenly? Therefore, we are looking for the LCM of 80 and 120. Find the LCM and then add this number of minutes to 4:00 P.M.

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Example 6: (continued) Begin with the prime factorization of 80 and 120: 80 = 24 5 120 = 23 3 5 Now select each prime factor, with the greatest exponent from each factorization. LCM = 24 3 5 = 16 3 5 = 240 Therefore, it will take 240 minutes, or 4 hours, for the movies to begin again at the same time. By adding 4 hours to 4:00 p.m., they will start together again at 8:00 p.m.

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