Adding Fraction With Different Denominator

Definition Common Denominator – When the fractions in an addition problem do not have the same denominators, rewrite the problem so that each fraction has the same denominator. Lowest Common Denominator – A common denominator is a number that can be divided evenly by all the denominators in a problem. The smallest number that can be divided evenly by all the denominators in a problem. Sometimes the largest denominator is a problem is the lowest common denominator.

Finding a Common Denominator Here are two ways of finding a common denominator when the largest denominator in addition problem doesn’t work. Multiply the denominators together. Go through the multiplication table of the largest denominator.

Example 1 2/5 + 3/4 = Multiply the denominators. 5 X 4 = is the lowest common denominator (LCD). Raise each fraction to 20ths. Add the new fractions. Change the answer to a mixed number. 2/5 = 8/20 + 3/4 =15/20 23/20 = 1 3/20

Example 2 2/3 + 5/6 + 3/4 = Go through the multiplication table of the 6’. 6 x 1 = 6, which cannot be divided by 4. 6 x 2 = 12, which can be divided by 3 and 4. Raise each fraction to 12ths. Add the new fractions. Change the answer to a mixed number and reduce. 2/3 = 8/12 5/6 =10/12 +3/4 = 9/12 27/12 = 2 3/12 = 2 1/4

Practice 3/5 + 2/3 = 3/7 + 1/3 = 1/4 + 3/5 + 7/10= 2/3 + 4/9 + 5/6 = 4 3/5 + 6 ¾ = 6 5/ /8 =