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SLIDE SHOW INSTRUCTIONS This presentation is completely under your control. This lesson will show only one step at a time, to see the next step you must press a key. (Actual names written on a key are in green) •TO STOP THE SLIDE SHOW: press ‘escape’ (Esc, top left of keyboard) •TO MOVE FORWARD: press the “spacebar” or Enter (PageDn, , , also work) •TO MOVE BACKWARD: press the key (PageUp, or also work) Factors & Multiples Prime numbers & Prime Factorization Copyright©2001 Lynda Greene Prime Numbers Copyright©2001 Lynda Greene Before we learn to find the Prime Factors of a number, we need to know what a Prime Number is. A Prime Number is a number that has only two FACTORS (numbers that will divide evenly with no remainders) These two factors are the original number and the number 1. Examples: Let’s look at the factors of several numbers. 2 3 4 5 6 7 1x2 1x3 1x4 2x2 1x5 1x6 2x3 1x7 These two numbers (4 and 6) have more than one set of factors, so they are called “composite numbers” The other numbers (2, 3, 5, and 7) have only one set of factors each, the number one (1) and the number itself. This means that 2, 3, 5, and 7 are Prime Numbers! Copyright©2001 Lynda Greene To find the Prime Factorization of any number, you must divide over and over by bigger and bigger prime numbers. But before you can do that, you need to know which numbers are prime. One way to do this is to have a list of prime numbers that you can refer to, but the easiest way (in the long run) is to memorize the prime numbers. (It helps to know your Prime Numbers later when you learn to reduce fractions, simplify square roots and factor polynomials) Memorizing tip #1: Look at the last digits in the middle two rows, they are almost exactly the same. (3rd row: last number changed, 4th row: has a number missing) Here is a list of the first 25 prime numbers. 2, 3, 5, 7, 11, 13, 17, 19, 23, 3 29, 9 31, 1 37, 7 41, 1 43, 3 47, 7 53, 3 59, 9 61, 1 67, 7 71, 1 73, 3 79, 9 83, 3 89, 9 1 977 Grouping Pattern: two two three 20’s 30’s 40’s 50’s 60’s 70’s 80’s ...pattern breaks down Memorizing tip #2: 2 primes in the 20’s, 2 in the 30’s, 3 in the 40’s, (then it repeats) 2 in the 50’s, 2 in the 60’s, 3 in the 70’s, (starts to repeat again) 2 in the 80’s Copyright©2001 Lynda Greene Practice Problems: (Hit enter to see the answers) Label each number below as prime, composite or neither 1) 73 5) 51 2) 87 6) 23 3) 77 7) 2 4) 29 8) 1 Answers: 1) prime 2) composite 3) composite 4) prime 5) composite 6) prime 7) prime 8) neither Copyright©2001 Lynda Greene Prime Factorization Copyright©2001 Lynda Greene Prime Factorization: Breaking a number up into the smallest possible pieces. These pieces are called “prime factors” and they are a group of “prime numbers” that when multiplied together are equal to the original number. Example: The prime factorization for 72 is: 2 x 2 x 2 x 3 x 3 or (23 x 32) These expressions multiply together to give you 72 and 2 and 3 are both prime numbers. Copyright©2001 Lynda Greene Example: Find the Prime Factorization for the number 36 1) General Steps: 2) Divide the number by 2 (since 2 is the first prime number in the prime number list) Divide the answer by 2 3) 4) Divide by 3, until it doesn’t divide evenly Divide by 5, 7, 11, ... (each of the prime numbers) *Keep using 2 until it doesn’t divide evenly anymore* STOP ! when you get a Prime Number on the bottom. Here is a list of the first 25 prime numbers. 2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31, 37, 41, 43, 47, 53, 59, 61, 67, 71, 73, 79, 83, 89, 97 Note: Many teachers insist on using this upside-down division symbol for Prime Factorization, but it is still plain old division, just write the answer underneath instead of on top. Divide by 2 Divide by 2 Divide by 3 2 ) 36 2 ) 18 3) 9 3 Prime Number STOP! Check: 2 x 2 x 3 x 3 = 36 You will get the original number back if your answer is correct. Can’t divide by 2 anymore, go to next Prime Number (3) The answer is made up of the prime numbers on the outside of the division symbols. They must be written with multiplication signs between them. ANSWER: 2 x 2 x 3 x 3 or 22 x 32 Copyright©2001 Lynda Greene Example: Find the Prime Factorization for the number 42 Divide 42 by 2, then divide the answers by 2 until they won’t divide evenly anymore, then divide by the next prime number (3, 5, 7, 11,...). Stop when you get a Prime Number on the bottom. Divide by 2 Divide by 3 2 ) 42 3 ) 21 7 Prime Number STOP! List of prime numbers. 2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31, 37, 41, 43, 47, 53, 59, 61, 67, 71, 73, 79, 83, 89, 97 ANSWER: 2 x 3 x 7 Can’t divide by 2 anymore, go to next Prime Number (3) Check your answer, 2 x 3 x 7=42 Copyright©2001 Lynda Greene Example: Find the Prime Factorization for the number 126 List of prime numbers. 2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31, 37, 41, 43, 47, 53, 59, 61, 67, 71, 73, 79, 83, 89, 97 Divide by 2 Divide by 3 2 ) 126 3 ) 63 3 ) 21 7 Divide by 3 Prime Number STOP! Can’t divide by 2 anymore, go to next Prime Number (3) ANSWER: 2 x 3 x 3 x 7 or 2 x 32 x 7 Check your answer, 2 x 3 x 3 x 7 = 126 Copyright©2001 Lynda Greene Example: Find the Prime Factorization for the number 220 List of prime numbers. 2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31, 37, 41, 43, 47, 53, 59, 61, 67, 71, 73, 79, 83, 89, 97 Divide by 2 Divide by 2 2 ) 220 2 ) 110 5 ) 55 11 Divide by 5 Prime Number STOP! Can’t divide by 2 anymore, go to next Prime Number (3) Can’t divide by 3 either, go to next Prime Number (5) ANSWER: 2 x 2 x 5 x 11 Check your answer, 2 x 2 x 5 x 11 = 220 Copyright©2001 Lynda Greene Example: Find the Prime Factorization for the number 273 List of prime numbers. 2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31, 37, 41, 43, 47, 53, 59, 61, 67, 71, 73, 79, 83, 89, 97 Divide by 3 Divide by 7 Prime Number STOP! 3 ) 273 7 ) 91 13 Can’t divide by 2 at all, go to next Prime Number (3) Can’t divide by 3 anymore, go to next Prime Number (5) Can’t divide by 5 either, go to next Prime Number (7) ANSWER: 3 x 7 x 13 Check your answer, 3 x 7 x 13 = 273 Copyright©2001 Lynda Greene Practice Problems: (Hit enter to see the answers) Find the prime factorization for the following numbers 1) 105 2) 72 3) 225 4) 135 5) 90 6) 63 Answers: 1) 3 x 5 x 7 2) 2 x 2 x 2 x 3 x 3 3) 3 x 3 x 5 x 5 4) 3 x 3 x 3 x 5 5) 2 x 3 x 3 x 5 6) 3 x 3 x 7 7) 2 x 7 x 11 8) 2 x 3 x 7 x 7 x 11 7) 154 8) 3234 Copyright©2001 Lynda Greene End of Tutorial Go to www.greenebox.com for more great math tutorials for your home computer Questions? send e-mail to: [email protected] Copyright©2001 Lynda Greene