Find an Unknown Side Length of a Rectangle
Find an Unknown Side Length of a Rectangle is a Grade 3 math skill from Eureka Math (second instance) reinforcing that the unknown side equals Area ÷ Known Side. The area multiplication formula Area = length × width contains an unknown factor when one dimension is missing. To isolate the unknown, divide the area by the known dimension: Unknown = Area ÷ Known Side Length. This is equivalent to solving ? × Known = Area. Third graders practice applying this relationship across varied problem contexts to build fluency with the inverse relationship of multiplication and division.
Key Concepts
To find an unknown side length of a rectangle, you can divide the area by the known side length. This is equivalent to solving for the unknown factor in the area multiplication equation. $$ \text{Unknown side length} = \frac{\text{Area}}{\text{Known side length}} $$ $$ \text{Known side length} \times ? = \text{Area} $$.
Common Questions
How do you find the unknown side of a rectangle when you know the area?
Divide the area by the known side length: Unknown Side = Area ÷ Known Side. This is because Area = length × width, and division isolates the unknown factor.
What equation helps you find an unknown side when area is known?
Known Side × ? = Area, which rearranges to ? = Area ÷ Known Side.
If a rectangle has area 72 square units and one side of 9 units, what is the unknown side?
72 ÷ 9 = 8 units.
What does it mean for division to be the inverse of multiplication in this context?
Multiplication builds the area from two side lengths. Division reverses that: given the area and one side, divide to recover the other side.
In which textbook is this version of Find an Unknown Side Length of a Rectangle taught?
This skill is taught in Eureka Math, Grade 3.