Step by step
The question
Question: find the area of this L-shape: 12 by 9, with a 5 by 4 corner cut out.
1 Split into rectangles
This shape is not a rectangle.
One line cuts it into two rectangles.
We call them top and bottom.
2 Find the missing sides
The top side is the whole 12, take away the 5 cut out.
12 minus 5 is 7.
12 − 5 = 7
The right side is the whole 9, take away the 4 cut out.
9 minus 4 is 5.
9 − 4 = 5
3 Area of each part
The top rectangle is 7 by 4.
7 times 4 is 28.
top = 7 × 4 = 28
The bottom rectangle is 12 by 5.
12 times 5 is 60.
bottom = 12 × 5 = 60
4 Add the parts
The whole area is the two parts added.
A = 28 + 60
28 plus 60 is 88.
So the area is 88 square units.
A = 88
5 Check
Let's check another way: the whole 12 by 9 rectangle, take away the corner.
12 × 9 − 5 × 4 = 108 − 20 = 88
We get 88 again.
So the area is right!
Let's look back.
First: split into rectangles.
Then: find the missing sides.
Then: area of each part.
Then: add the parts.
Last: check.
Answer
A = 88