Adding Fractions - Uncommon Denominators 7/9/25

Can you add fractions with different denominators?

No, you need a common denominator first

What is LCM?

Least Common Multiple – the smallest number both denominators can go into

Why do we need the LCM?

To make the denominators match so we can add

Example LCM of 8 and 12

LCM = 24

Multiples of 8

8, 16, 24, 32

Multiples of 12

12, 24, 36

Example problem: 3/8 + 5/12

Step 1: Find LCM → 24

Convert 3/8 to have denominator 24

3/8 × 3 = 9/24

Convert 5/12 to have denominator 24

5/12 × 2 = 10/24

Now add: 9/24 + 10/24 =

19/24

Final answer

19/24

Do you simplify 19/24?

No, it’s already in simplest form

Another example: 2/5 + 1/4

LCM of 5 and 4 = 20

Convert 2/5 →

8/20

Convert 1/4 →

5/20

Now add: 8/20 + 5/20 =

13/20

Final answer

13/20

Important tip: Which numbers do you add?

Add only the top numbers (numerators)

Does the denominator change?

No, the denominator stays the same

What do you check at the end?

Can you simplify the final answer?