Adding and subtracting fractions with unlike denominators
Adding and subtracting fractions with unlike denominators
In this section, we will learn how to add and subtract fractions with unlike denominators. First, we will practice finding the lowest common denominator for a pair of fractions by determining the lowest common multiple of the denominators. The divisibility rules learned in previous section can be used to find the multiples of our denominators. Next, we will practice writing equivalent fractions. When writing equivalent fractions with common denominators, the numerator and denominator in each fraction are both multiplied by the same number. As shown in sections on adding/subtracting fractions with like denominators, we will write our answers in lowest terms by first finding the greatest common factor (GCF) of both the numerator and denominator in our equivalent fraction and then dividing both the numerator and denominator by this GCF.
Basic concepts:
 Divisibility rules
 Determining Common Factors
Lessons

1.

e)
How to subtract frations?
• subtracting with improper fractions
• subtracting with mixed numbers

2.
Determine the lowest common denominator (LCD) for the pair of fractions using multiples.

3.
Determine the lowest common denominator (LCD) for the pair of fractions using multiples. Then, determine equivalent fractions for the pair of fractions using the LCD.

4.
Calculate using the four square method. Write the answer in the lowest terms.