basic geometric properties
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Basic Geometric Properties
Unit 3 Lesson 7 Finding Area of Figures made up of Triangles and
Parallelograms
Example 1: Find the area of this shape
12 cm
3 cm
8 cm
5 cm
We can split it into 2 rectangles, A and B
A
B
Rectangle A has area 5 8 = 40 cm2For rectangle B, we have to find another length.
x
Rectangle B has area 7 3 = 21 cm2
So total area = 40 + 21 = 61 cm2
40 cm2
Can you see why it is x = 7 cm?
21 cm2
Find the area
6 cm
5 cm
7 cm
12 cm
A B
CWe can split it into 3 rectangles, A, B and C
X3 cm
Because of the symmetry of the shape, can you see why these two lengths must be 3 cm each?Can you also see why the dotted lines are y = 2 cm each
y
Rectangle AArea =Rectangle B
Rectangle CArea =
Total area =
21cm2 21cm2
12cm2
You Try1.
2.
3. 4.
6cm
4cm
3cm
9cm
3cm
2cm
7cm
5cm
7cm
4cm6cm
3cm 6cm
7cm
4cm2cm
12cm2
6cm2
12cm2
8cm2
Steps to success Split into small rectangles Calculate any missing lengthsWork out area of each rectangleAdd for total areaRemember units (square!)
Area of Compound Shapes13 cm
8cm
9 cm
3cmx
y
Area x = 8 X 9 = 72 cm2
?4cm
Area y = 3 X 4 = 12cm2
Total Area = 72 + 12
= 84
cm2
14cm
8 cm
10 cmx
y
Area x = 10 X 8 = 80 cm2
4 cm
Area y = ½ X 8 X 4= 16 cm2
Total Area = 80 + 16
= 96cm2
Other Compound Shapes
Area of Composite Shapes Find the area of all three triangles.
Plan:
Area of a Triangle
½(b)(h)
½(24)
6 in
5 in
4 in
12 in
½(6)(4)
One triangle is 12 square inches
Since 3 triangles are the same.
3 x 12 = 36 square inches
10m
3m
7m
1m
Big Area = 3 X 10 = 30m2
White Area = 7 X 1 = 7m2
Green Area = 30 – 7 = 23m2
Find the area of the green only
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