Adding and Subtracting Fractions - with Different Denominators

To add or subtract fractions with different denominators, we need to do some extra steps. The general approach is discussed below. We will go over few examples in this lesson to make sure you get comfortable with the procedure.

 Steps to add/subtract fractions with different denominators Step 1: Given two unlike fractions where . Step 2: Make the denominators the same by finding the Least Common Multiple (LCM) of their denominators. Step 3: Rewrite each fraction into its equivalent fraction with a denominator of . Step 4: Now, add or subtract the "new" fractions from step 3. Always reduce the answer to its lowest terms.

Example 1: Add the fractions with different denominators .

The two fractions have denominators that are not equal. We need to make them equal by finding their Least Common Multiple that will serve as their Least Common Denominator (LCD).

Start by listing the multiples of each denominator, and identify the least number that is common to both of them.

 multiples of each denominator common denominator multiples of multiples of

The first fraction already has a denominator equal to the LCM = 15, and so we will leave it alone.

The second fraction requires some adjusting to make its denominator equal to 15. Do that by multiplying its numerator and denominator by the number 3.

• Once their denominators are equal, add the fractions by adding their numerators and then copying the common denominator.

The fraction is our final answer because it is already in its lowest term.

Example 2: Add the fractions with different denominators .

We can't add the two fractions just yet because they have different denominators, namely 5 and 9. Begin by listing their multiples and pick the smallest number that is common to both. This will become their common denominator.

 multiples of each denominator common denominator multiples of multiples of

Now, convert each fraction to an equivalent fraction with the LCM as its denominator, then proceed with regular addition.

Look for the opportunity to reduce the answer to its lowest term. The numerator and denominator of is divisible by 3.

Example 3: Add the fractions with different denominators .

Sometimes there is no "need" to find the least common denominator by list method. We can immediately find it whenever the two numbers are both prime.

• A prime number is a number divisible only by 1 and itself.

Observe that the denominators are primes. The LCD will simply be their product, that is, .

Example 4: Add the fractions with different denominators .

Solution:

Find the least common multiple of the the denominators.

 multiples of each denominator common denominator multiples of multiples of

Make the necessary adjustments in the denominator and proceed as usual. Reduce your final answer to the lowest term.

Example 5: Add the fractions with different denominators .

Solution:

Since the denominators are both prime numbers, the least common denominator will be their product.

• LCD = 11 x 13 = 143

Convert the current denominators of the two fractions into the LCD, and proceed with regular addition.

Example 6: Subtract the fractions with different denominators .

To subtract these fractions with unequal denominators is very similar to addition.

Make their denominators equal using the concept of least common multiple. Then subtract their numerators accordingly.

 multiples of each denominator common denominator multiples of multiples of

Rewrite each fraction to its equivalent fraction with a denominator equal to the LCM = 30, then subtract their numerators. Make sure to reduce your answer to the lowest term.

Example 7: Subtract the fractions with different denominators .

Since the denominators are both prime numbers, their LCM is just their product, thus .

Example 8: Subtract the fractions with different denominators .

Solution:

Find the least common denominator by determining the LCM of the denominators.

 multiples of each denominator common denominator multiples of multiples of

Rewrite the two fractions with a common denominator equal to the LCM = 42. Subtract their numerators, and reduce the answer to the lowest term if possible.

Example 9: Subtract the fractions with different denominators .

Solution:

Find the least common denominator by solving for the least common multiple of the denominators.

 multiples of each denominator common denominator multiples of multiples of

We make adjustments on the existing fractions to make their denominator equal to the LCD = 40. After doing so, subtract their numerators and copy the common denominator.

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