# Lesson: Least Common Multiple

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### Lesson Objective

SWBAT identify multiples and the least common multiple of a set of whole numbers.

### Lesson Plan

 State Standard: 5.NSO-N.5, 5.NSO-C.13 Standard Name: Least Common Multiple Objective: SWBAT identify multiples and the least common multiple of a set of whole numbers. Essential Question: How does mental math help us carry out more difficult math tasks? Word Wall Words: Skip Count LCM Do Now: Students will write out the skip counting chants for 3, 4, 6, 7, 8, 9, 11, 12 Opening: We practiced our skip counting in our Do Now today because we are going to need it for learning about least common multiple, or LCM. Who can remember when else we have used skip counting to help us? That’s right, when we were doing our mental math multiplication facts. Well today, we are going to need to know our multiplication facts, or multiples, which is what we are doing when we are skip counting, in order to find the LCM. Direct Instruction: Today we are learning how to find the Least Common Multiple (LCM) between a set of 2 numbers, because this will help us when we start adding and subtracting fractions.   Let’s look at the numbers 3 and 4. I’m going to start listing out the multiples by skip counting, but I’m not going to do the whole chant. I usually like to do the first 4 or 5 multiples of each number to see if I find any that are the same before I keep going.   3: 6, 9, 12, 15, 18 4: 8, 12   On this one, I only had to list out 2 multiples of 4 because I already found a common multiple. Is 12 a multiple of both 3 and 4? Yes, so it could be called a common multiple. For fun, let’s see if there are any other common multiples…   3: 6, 9, 12, 15, 18, 21, 24, 27, 30 4: 8, 12, 16, 20, 24, 28, 32   We see that 24 is also a common multiple. A set of numbers might have several common multiples, but we are trying to find the Least common multiple. Therefore, even though 12 and 24 are common multiples, 12 is the least common multiple.   What if I had 2 and 4. First, write out the  multiples of 2: 2, 4 – look, I can already stop because if 4 is a multiple of 2, and my other number is 4, isn’t 4 and 4 the same? Yes. I could write out multiples of 4 and we would probably find other common ones, but I’m ONLY concerned about the lease or lowest common multiple. Therefore in this case, 4 is the LCM. Guided Practice: Let’s try a few together.   2 and 3 4 and 5 5 and 6   CHALLENGE: 7 and 8   On our whiteboards, we are going to practice. Remember to list out all the multiples by skip counting, then circle the smallest one they have in common. Students will think, write, show. Independent Practice: Students will complete a worksheet with 20 sets of numbers ranging from 2-12 and find LCM by writing out the skip count of each number and then circling the LCM. Math Journal: How is skip counting used to help find the least common multiple? Closing: Today we learned how to find the LCM by skip counting and finding the least common multiple between a set of two numbers. Center Options: Drill and practice with skip counting. Extra practice finding LCM on a worksheet with different sets of numbers. Have students roll dice and see who can find the LCM first between the 2 numbers rolled. Differentiation: Low: have students get in partners and listen to skip counting chants in the reading center and practice them together.   High: give 3 numbers to find the LCM for.

### Lesson Resources

 LCM   Smart Board 1,207 Word Wall Word Cards Fractions   Vocabulary 880 Skip Counting Powerpoint   Smart Board 746 LCM Worksheet   Classwork 1,551