YOU TRY 5 Subtract 10 11 7 12 vertically. 3 Add and Subtract Fractions with Large Denominators If we are asked to add or subtract fractions with large denominators, remember that we can fi nd the least common denominator using prime factorization. Add 19 60 22 45 . Find the LCD using prime factorization. Solution Step 1: Find the LCD of 19 60 and 22 45 by fi rst fi nding the prime factorizations of 60 and 45. 60 2 2 3 5 45 3 3 5 The LCD is 2 2 3 3 5 180. Step 2: Rewrite each fraction with the LCD. 19 60 3 3 57 180 Step 3: Add. Equivalent fractions 19 60 22 45 57 180 88 180 22 45 4 4 88 180 57 88 180 145 180 Equivalent fractions Step 4: Ask yourself, “Is 145 180 in lowest terms?” No! Write it in lowest terms. 145 180 145 5 180 5 29 36 The fi nal answer is 29 36 . EXAMPLE 6 YOU TRY 6 Add 5 18 3 28 . Find the LCD using prime factorization. Note When the denominators are large, making a list of multiples of the larger number is also a good way to find the LCD. 248 CHAPTER 4 Adding and Subtracting Fractions www.mhhe.com/messersmith
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