Finding the LCM of Expressions
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Finding the LCM of Expressions. When adding and subtracting rational expressions without common denominators, the Lowest Common Denominator ( LCD ) must be found. The LCD of the rational expressions is the same as the Least Common Multiple ( LCM ) of the two denominators.

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Finding the LCM of Expressions

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Finding the lcm of expressions

Finding the LCM of Expressions

  • When adding and subtracting rational expressions without common denominators, the Lowest Common Denominator (LCD) must be found.

  • The LCD of the rational expressions is the same as the Least Common Multiple (LCM) of the two denominators.

  • The LCM of two integers a and b is the smallest positive integer that botha and b will divide into evenly.


Finding the lcm of expressions

  • Example 1

Find the LCM of 3 and 5.

The smallest number that both 3 and 5 will divide into evenly is 15.


Finding the lcm of expressions

  • Example 2

Find the LCM of 4 and 6.

The smallest number that both 4 and 6 will divide into evenly is 12.

Note that both 4 and 6 will divide evenly into 24, but 12 is the smallest such number, and is the LCM.


Finding the lcm of expressions

  • Example 3

Find the LCM of 36 and 45.

This problem is more difficult and we will need another procedure rather than just working in our head.

Write the prime factorization of each number. We will use factor trees to do this.


Finding the lcm of expressions

Use the prime factorizations to “build” the LCM.

Start with the smallest prime used in either factorization.

The largest exponent on 2 in either factorization is …


Finding the lcm of expressions

Use the prime factorizations to “build” the LCM.

Start with the smallest prime used in either factorization.

The largest exponent on 2 in either factorization is …


Finding the lcm of expressions

The next prime is 3.

The largest exponent on 3 in either factorization is …


Finding the lcm of expressions

The next prime is 3.

The largest exponent on 3 in either factorization is …


Finding the lcm of expressions

The last prime is 5.

The largest exponent on 5 in either factorization is 1.


Finding the lcm of expressions

Multiply …

The LMC of 36 and 45 is 180.

  • The same pattern used in this problem can be used with variable expressions.


Finding the lcm of expressions

  • Example 4

Find the LCM of the two expressions:

Write the prime factorization of each.


Finding the lcm of expressions

Remember to use the largest exponent.


Finding the lcm of expressions

  • Example 5

Find the LCM of the two expressions:

Write the prime factorization of each.


Finding the lcm of expressions

END OF PRESENTATION


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