Adding & Subtracting Areas Flashcards

All 6 cards in this deck

What is a compound (composite) shape?

A shape made by joining two or more basic shapes, such as rectangles and triangles.

What are the steps to find the area of a compound shape by adding?

e.g. an L-shape split into a 5 cm by 2 cm rectangle and a 3 cm by 4 cm rectangle

1. Split the shape into rectangles (and triangles)
2. Work out each area: $5\times2=10$ and $3\times4=12$
3. Add them: $10+12=22$ cm$^2$

What are the steps to find the area of a shaded region?

e.g. a 10 cm by 6 cm rectangle with a 4 cm by 3 cm rectangle cut out of it

1. Area of the whole shape: $10\times6=60$
2. Area of the unshaded part: $4\times3=12$
3. Subtract: $60-12=48$ cm$^2$

True or false? The area of a compound shape equals the sum of the areas of the parts it is split into.

True. Splitting a shape does not change the total area.

True or false? The perimeter of a shape made from two rectangles joined together is the sum of the two rectangles' perimeters.

False. The joined edges are inside the shape, so they are not part of its perimeter.

True or false? An L-shape's area can be found by subtracting a rectangle from a larger surrounding rectangle.

True. Adding parts or subtracting a missing piece both give the same area.