Calculate Perimeter and Find an Unknown Side Length
This Grade 4 Eureka Math skill teaches students to calculate the perimeter of a rectangle using P = 2 times (l + w), and to find an unknown side length when the perimeter and one dimension are known. For example, a rectangle with length 10 cm and width 4 cm has perimeter 2 times (10 + 4) = 28 cm. To find the width when P = 30 m and l = 9 m, students use w = P/2 minus l = 15 minus 9 = 6 m. This skill from Chapter 9 of Eureka Math Grade 4 connects perimeter formulas to multiplicative comparison reasoning.
Key Concepts
The perimeter of a rectangle is calculated using the formula $P = 2 \times (l + w)$, where $l$ is the length and $w$ is the width. To find an unknown side length (e.g., width $w$) when the perimeter $P$ and length $l$ are known, you can use the formula: $$w = \frac{P}{2} l$$.
Common Questions
What is the formula for the perimeter of a rectangle?
P = 2 times (l + w), where l is the length and w is the width. Add the length and width, then multiply by 2.
What is the perimeter of a rectangle with length 10 cm and width 4 cm?
P = 2 times (10 + 4) = 2 times 14 = 28 cm.
How do you find the width when perimeter and length are known?
Use w = P/2 minus l. For P = 30 m and l = 9 m: w = 15 minus 9 = 6 m.
How do you find an unknown side length from the perimeter formula?
First, divide the perimeter by 2 to find l + w. Then subtract the known dimension to find the unknown one.
What are the units for perimeter?
Perimeter is measured in linear units such as centimeters, meters, or inches. It represents the total distance around the outside of the rectangle.