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.
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.