Find Area of Composite Figures Using Subtraction
Find Area of Composite Figures Using Subtraction is a Grade 3 math skill from Eureka Math teaching an alternative decomposition strategy. Instead of breaking a composite figure into smaller pieces and adding, enclose the figure in a large rectangle and subtract the missing rectangular portion: A_composite = A_large_rectangle - A_missing_rectangle. This method is especially useful when the missing piece is easier to identify than the remaining parts. Third graders practice both addition and subtraction strategies for composite area, choosing the most efficient approach.
Key Concepts
To find the area of a composite figure, you can calculate the area of the large, enclosing rectangle and subtract the area of the missing rectangular portion. $$A {\text{composite}} = A {\text{large rectangle}} A {\text{missing rectangle}}$$.
Common Questions
How do you find the area of a composite figure using subtraction?
Enclose the entire composite figure in the smallest possible rectangle. Calculate the area of that rectangle. Subtract the area of the missing rectangular cutout: A_composite = A_large - A_missing.
When should you use subtraction rather than addition to find composite area?
Use subtraction when the figure is a rectangle with a corner or section cut out. It is easier to identify and subtract the missing piece than to decompose the remaining figure into multiple parts.
Give an example of using subtraction to find composite area.
A large 8 × 6 rectangle with a 2 × 3 piece removed from one corner: A_large = 48, A_missing = 6. A_composite = 48 - 6 = 42 square units.
What is the formula for finding composite area using subtraction?
A_composite = A_large_rectangle - A_missing_rectangle.
In which textbook is Find Area of Composite Figures Using Subtraction taught?
This skill is taught in Eureka Math, Grade 3.