# Learn how to add fractions: theory, steps and a practical example. Adding fractions with like or unlike denominators

## How to: Adding ordinary (simple, common) math fractions. Steps.

There are two cases regarding the denominators when we add ordinary fractions:

• A. the fractions have like denominators;
• B. the fractions have unlike denominators.

### A. How to add ordinary fractions that have like denominators?

• Simply add the numerators of the fractions.
• The denominator of the resulting fraction will be the common denominator of the fractions.
• Reduce the resulting fraction.

### An example of adding ordinary fractions that have like denominators, with explanations

• #### 3/18 + 4/18 + 5/18 = (3 + 4 + 5)/18 = 12/18;

• We simply added the numerators of the fractions: 3 + 4 + 5 = 12;
• The denominator of the resulting fraction is: 18;

### B. To add fractions with different denominators (unlike denominators), build up the fractions to the same denominator. How is it done?

• #### 1. Reduce the fractions to the lowest terms (simplifying).

• Factor the numerator and the denominator of each fraction down to prime factors (prime factorization).
• #### Factor numbers online down to their prime factors.

• Calculate GCF, the greatest common factor (also called GCD, greatest common divisor, HCF, greatest common factor) of each fraction's numerator and denominator.
• GCF is the product of all the unique common prime factors of the numerator and the denominator, taken by the lowest exponents.
• #### Calculate online the greatest common factor, GCF.

• Divide the numerator and the denominator of each fraction by their greatest common factor, GCF - after this operation the fraction is reduced to its lowest terms equivalent.

• #### 2. Calculate the least common multiple, LCM, of all the fractions' new denominators:

• LCM is going to be the common denominator of the added fractions.
• Factor all the new denominators of the reduced fractions (run the prime factorization).
• The least common multiple, LCM, is the product of all the unique prime factors of the denominators, taken by the largest exponents.

• #### 3. Calculate each fraction's expanding number:

• The expanding number is the non-zero number that will be used to multiply both the numerator and the denominator of each fraction, in order to build all the fractions up to the same common denominator.
• Divide the least common multiple, LCM, calculated above, by each fraction's denominator, in order to calculate each fraction's expanding number.
• #### 4. Expand each fraction:

• Multiply each fraction's both numerator and denominator by expanding number.
• At this point, fractions are built up to the same denominator.
• #### 5. Add the fractions:

• In order to add all the fractions simply add all the fractions' numerators.
• The end fraction will have as a denominator the least common multiple, LCM, calculated above.

### An example of adding fractions that have different denominators (unlike denominators), step by step explanations

• #### 2. Calculate the least common multiple, LCM, of all the fractions' new denominators

• Factor all the denominators down to their prime factors then multiply ALL the unique prime factors found, by the largest exponents.

• #### 3. Calculate each fraction's expanding number:

• Divide the least common multiple LCM by each fraction's denominator.

• #### 4. Expand each fraction:

• Multiply both the numerator and the denominator of each fraction by their expanding number.

• #### 5. Add the fractions:

• Simply add fractions' numerators. The denominator = LCM.

• #### 6. Reduce the end fraction to the lowest terms, if needed.

• In this particular case it was no longer needed to reduce the fraction, since the numerator and the denominator are coprime numbers (prime to each other, no other common factors than 1).
• #### 7. Extra step - rewrite the end fraction:

• Since the final fraction is an improper one (also called a top-heavy fraction), in other words the absolute value of the numerator is larger than the absolute value of the denominator, it can be written as a mixed number (also called a mixed fraction):