7th Grade · The Number System
Common Denominator
7.NS.A.1d
Definition
A shared multiple of the denominators of two or more fractions. The Least Common Denominator (LCD) is the smallest such multiple.
Teach it
Where it is taught, with guided notes, practice, homework, an exit ticket and an interactive Teacher Deck.
Examples
- For 1/3 + 1/4, the LCD of 3 and 4 is 12.
- 1/3 = 4/12 and 1/4 = 3/12, so 4/12 + 3/12 = 7/12.
Key rule
You must have the same denominator before adding or subtracting fractions.
Memory hook
Same bottom before you add.