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book on your desk.
1.
2.
Area: Triangles and
Trapezoids
Lesson 10-2 p.509
Triangles

When finding the area of triangles,
remember that a triangle is half of a
parallelogram.
Triangles

Now you can see why the formula for the
area of a triangle makes sense:

A = bh
2
or
A=
1
2
bh
Triangles

Let’s look at an example:
5 feet
4 feet
A = bh
2
= 4 (5) =
2
or 10 ft2
20
2
Try This

Find the areas:
3 ft.
12 in.
5 mi.
7 miles
6 miles
Try This

Find the areas:
3 ft.
12 in.
216 in2 or
1.5 ft2
5 mi.
7 miles
6 miles
Try This

Find the areas:
3 ft.
12 in.
216 in2 or
1.5 ft2
7 miles
5 mi.
6 miles
15 mi2
Trapezoids


Trapezoids have different formula. It
looks like this:
A=
1
2
h (b1 + b2)
or A = h(b1 + b2)
2
b1
h
b2
Trapezoids
or A = h(b1 + b2)
2
2
Note that there are 2 bases—base 1 and
base 2. The formula takes the average of
the two bases multiplied by the height.

A=
1
h (b1 + b2)
b1
h
b2
Trapezoids

Let’s try an example:
4 ft
3.5 ft
6 feet

A = h (b1 + b2)
2
A = 3.5 (4 + 6)
2
A = 3.5 (5) = 17.5 ft2
Try This

Find the area:
24 mm
12 mm
33 mm
Try This

Find the area:
24 mm
12 mm
33 mm
342 mm2
Try This

Find the area:
2 feet
4.5 feet
4 feet
Try This

Find the area:
2 feet
4.5 feet
4 feet
13 ft2
Example

Some shapes look unusual, but when you
remember just 3 formulas, you can
calculate the area.
Which two shapes do you
see in the figure at the left?
Example

There is a square (parallelogram) and a
triangle. To find the area, first find the
area of the parallelogram and then add it
to the area of the triangle.
Example
The square has an area of A = bh or 3 (3) or 9 ft2.
The triangle has an area of A = ½ bh or ½ (3) (2) or 3 ft2.
The total area if 9 + 3 or 12 ft2
3 ft
2 ft
Try This

Find the area:
6 cm
6 cm
4 cm
8 cm
Try This

Here is a hint:
6 cm
6 cm
4 cm
8 cm
Try This

Here is a hint:
6 cm
4 cm
8 cm
For the rectangle:
A = bh
A = 6 (4) = 24 cm2
6 cm For the trapezoid:
A = ½ h (b1 + b2)
A = ½ (2) (4 + 6)
A = 10 cm2
Total area =
24 + 10 = 34 cm2
Try This

Is there another way to divide this up?
6 cm
6 cm
4 cm
8 cm
Try This

How about like this:
6 cm
6 cm
4 cm
8 cm
Try This

How about like this:
6 cm
For the rectangle:
A = bh = 8 (4) = 32 cm2
6 cm
4 cm
8 cm
For the triangle:
A = ½ bh = ½ (2) (2)
= 2 cm2
The total area is 32 + 2 or
34 cm2
Agenda
PA#38
Pp.512-513
#8-16 even,
17, 19
Please
start
Bellwork #
HW, red pen book
on desk.
Agenda
PA#
Workbook
pp.83 & 84
Benchmark 3 is
Friday.