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Adding & Subtracting Fractions Teacher Notes: Use this presentation to teach the steps for adding and subtracting fractions with unlike denominators. It is assumed that students have a thorough understanding of the process for making equivalent fractions. CLICK HERE to view a demonstration with opportunities for students to practice making equivalent fractions.

Steps List the multiples of both denominators. Find the least common multiple (LCM). Write new fractions with the LCM as the new denominator. Find the factor you multiply by to get from your original denominator to your new denominator. Use that same factor, and multiply it by your original numerator to get a new numerator.

1 2 We need a common denominator to add these fractions. 3 7 +

1 2 + We need a common denominator to add these fractions. 3 7 Count by 2's 2, 4, 6, 8, 10, 12, 14, 16, 18, 20 Count by 7's 7, 14, 21, 28, 35…

Count by 2's 2, 4, 6, 8, 10, 12, 14, 16, 18, 20 Count by 7's 7, 14, 21, 28, 35… The first number IN COMMON that appears on both lists becomes the common denominator

1 2 3 7 7 6 13 = = + 7 + 6 = 13 x 7 X 7 x 2 x 2 Make equivalent fractions. = X 7 x 2 6 3 7 Add the numerators = + 7 + 6 = 13 x 2 13

You MUST have a common denominator to add fractions! ADDING MIXED NUMBERS 3 ½ 5 1/3 X 6 5 3/6 2/6 2 + 11 5/6 You MUST have a common denominator to add fractions! Find the least common multiple of 3 and 2. This number is 6. Once you have the common denominator, add the fractions, then add the whole numbers! 6th Grade HomePage

3 11 - We need a common denominator to subtract a fraction from another. 1 5 Count by 5's 5, 10, 15, 20, 25, 30, 35, 40, 45, 50, 55 Count by 11's 11, 22, 33, 44, 55, 66, 77…

3 11 1 5 15 11 4 15 - 11 = 4 = = - x 5 x 5 x 11 x 11 Make equivalent fractions. = x 5 11 1 5 x 11 Subtract. = 15 - 11 = 4 - x 11 4

SUBTRACTING FRACTIONS WHILE BORROWING + 4/4 𝟏 𝟐 𝟑 𝟒 𝟐 𝟒 2 𝟑 𝟒 4 2 4 2 3 4 2 1 6/4 2 3/4 X First…Find a common denominator. 2 3/4 You cannot subtract 3 from 2, so you MUST borrow! ANSWER! Find the least common multiple of 4 and 2. This number is 4.