Holt Lesson

1. 9-4 Area of Triangles and Trapezoids
Course 2
Warm UpWarm Up
Problem of the DayProblem of the Day
Lesson PresentationLesson Presentation

2. Warm Up
Evaluate.
24
19.44
68
Course 2
9-4 Area of Triangles and Trapezoids
1
2 · 6 · 8
1
2
· 5.4 · 7.2
3. 4(7 + 10)
4. 3.5(12 + 8.2)
1.
70.7
2.

3. Problem of the Day
(5.5)(1.5) = 8.25 in2
Course 2
9-4 Area of Triangles and Trapezoids
4 in.
1.5 in.
7 in.
An isosceles trapezoid has bases of 7 in. and 4 in.
and height 1.5 in. Find its area by using only the
formula for the area of a parallelogram.

4. Learn to find the area of triangles and
trapezoids.
Area of Triangles and Trapezoids
Course 2
9-4 Area of Triangles and Trapezoids

5. Course 2
9-4 Area of Triangles and Trapezoids
AREA OF A TRIANGLE
h
b
A =1
2
bh
The area A of a triangle
is half the product of its
base b and its height h.

6. Find the area of the triangle.
Additional Example 1A: Finding the Area of a
Triangle
Course 2
9-4 Area of Triangles and Trapezoids
5
8
A =
1
2
bh Use the formula.
A =
1
2
(8 · 5) Substitute 8 for b and
5 for h.
A = 20
The area of the triangle is 20 square units.

7. Find the area of the triangle.
Additional Example 1B: Finding the Area of a
Triangle
Course 2
9-4 Area of Triangles and Trapezoids
9
A =1
2
bh Use the formula.
A =1
2
(9 · 12) Substitute 9 for b and
12 for h.
A = 54
The area of the triangle is 54
square units.
12

8. Find the area of the triangle.
Course 2
9-4 Area of Triangles and Trapezoids
A =
1
2
bh Use the formula.
A =
1
2
(6 · 9) Substitute 6 for b and
9 for h.
A = 27
The area of the triangle is 27 square units.
Check It Out: Example 1A
6
9

9. Find the area of the triangle.
Course 2
9-4 Area of Triangles and Trapezoids
A =
1
2
bh Use the formula.
A =
1
2
(7 · 10) Substitute 7 for b and
10 for h.
A = 35
The area of the triangle is 35 square units.
Check It Out: Example 1B
10
7

10. Course 2
9-4 Area of Triangles and Trapezoids
The two parallel sides of a trapezoid are its
bases, b1 and b2. The height of a trapezoid
is the perpendicular distance between the
bases.
h
b1
b2

11. Course 2
9-4 Area of Triangles and Trapezoids
AREA OF A TRAPEZOID
h
b1
A = 1
2
h(b1 + b2)
The area of a trapezoid
is half its height
multiplied by the sum of
its two bases.
b2
In the term b1, the number 1 is called a
subscript. It is read as “b-one” or “b sub-one.”
Reading Math

12. Find the area of the trapezoid.
Additional Example 2A: Finding the Area of a
Trapezoid
Course 2
9-4 Area of Triangles and Trapezoids
5 in.
6 in.
9 in.
A =1
2
h(b1 + b2) Use the formula.
A = 1
2
· 6(5 + 9)
A =1
2
· 6(14)
A = 42
The area of the trapezoid is 42 in2
.
Substitute.
Multiply.
Add.

13. Find the area of the trapezoid.
Additional Example 2B: Finding the Area of a
Trapezoid
Course 2
9-4 Area of Triangles and Trapezoids
A =1
2
h(b1 + b2) Use the formula.
A = 1
2
· 7(12 + 16)
A =1
2
· 7(28)
A = 98
The area of the trapezoid is 98 cm2
.
Substitute.
Multiply.
Add.
12 cm
16 cm
7 cm

14. Find the area of the trapezoid.
Course 2
9-4 Area of Triangles and Trapezoids
A =1
2
h(b1 + b2) Use the formula.
A = 1
2
· 6(11 + 4)
A =1
2
· 6(15)
A = 45
The area of the trapezoid is 45 in2
.
Substitute.
Multiply.
Add.
Check It Out: Example 2A
4 in.
6 in.
11 in.

15. Find the area of the trapezoid.
Course 2
9-4 Area of Triangles and Trapezoids
A =1
2
h(b1 + b2) Use the formula.
A = 1
2
· 9(5 + 16)
A =1
2
· 9(21)
A = 94.5
The area of the trapezoid is 94.5 cm2
.
Substitute.
Multiply.
Add.
Check It Out: Example 2B
16 cm
5 cm
9 cm

16. The state of Wisconsin is shaped somewhat
like a trapezoid. What is the approximate area
of the state?
Additional Example 3: Geometry Application
Course 2
9-4 Area of Triangles and Trapezoids
A =1
2
h(b1 + b2) Use the formula.
A = 1
2
· 300(153 + 220)
A =1
2
· 300(373)
A = 55,950
The area of Wisconsin is approximately
56,000 square miles.
Substitute.
Multiply.
Add.

17. A large preserve has a similar trapezoid
shape. What is the approximate area of the
preserve?
Check It Out: Example 3
Course 2
9-4 Area of Triangles and Trapezoids
232 mi
116 mi
A =1
2
h(b1 + b2) Use the formula.
A = 1
2
· 129(232 + 116)
A =1
2
· 129(347)
A = 22,446
The area of the preserve is approximately
22,446 square miles.
Substitute.
Multiply.
Add.
129 mi

18. Lesson Quiz
Find the area of each figure.
1.
3.
9 in2
45 ft2
Insert Lesson Title Here
60 ft2
87.5 ft2
Course 2
9-4 Area of Triangles and Trapezoids
2.
4.
4. What is the height of a triangle with area
36 cm2
and a base 9 cm? 8 cm