1. EXAMPLE 1
Finding Area and Perimeter of a Triangle
Find the area and perimeter of the triangle.
A = bh 1 2
P = a + b + c
= (14) (12) 1 2 =
= 84 in.2
= 42 in.

2. Finding the Area of a Triangle
EXAMPLE 2
Finding the Area of a Triangle
Sailboats
Find the area of the sail.
A = bh 1 2
Write area formula.
= (171) (134) 1 2
Substitute values.
= 11,457
Multiply.
The area of the sail is 11,457 square inches.
ANSWER

3. GUIDED PRACTICE
for Examples 1 and 2
A triangle’s height is 9 inches and its base is 10 inches. Find its area. 1.
The area of the triangle is 45 in.2.
ANSWER

4. GUIDED PRACTICE
for Examples 1 and 2
A triangle’s side lengths are 4 feet, 6 feet, and 2.5 feet. Find its perimeter.
The perimeter of the triangle is 12.5 ft.
ANSWER

5. Standardized Test Practice
EXAMPLE 3
Standardized Test Practice
SOLUTION
A = lw
Write area formula.
63 = 9w
Substitute values.
63 = 93 9
Divide each side by 9.
7 = w
Simplify.
ANSWER
The width is 7 feet. The correct answer is A.

6. Finding an Unknown Length
EXAMPLE 4
Finding an Unknown Length
The rectangle at the right has a perimeter of 20 inches. Find the length of the rectangle.
p = 2l + 2w
Write perimeter formula.
20 = 2l + 2 (4)
Substitute 20 for P and 4 for w.
20 = 2l + 8
Multiply.
20 – 8 = 2l + 8 – 8
Subtract 8 from each side.
12 = 2l
Simplify.

7. Finding an Unknown Length
EXAMPLE 4
Finding an Unknown Length 2l 2 12 =
Divide each side by 2.
6 = l
Simplify.
ANSWER
The length of the rectangle is 6 inches.

8. EXAMPLE 5
Using an Area Formula
Zoo Exhibit
A polar bear exhibit includes a rectangular pool and rocks. Find the area of the region with rocks.
SOLUTION
Let r be the area of the region with rocks.

9. The area of the region with rocks is 1600 square feet. ANSWER
EXAMPLE 5
Using an Area Formula
= r
2240 = r
Write an algebraic model.
2240 – 640 = 640 – r
Subtract 640 from each side.
1600 = r
Simplify.
The area of the region with rocks is 1600 square feet.
ANSWER

10. GUIDED PRACTICE
for Examples 3, 4 and 5
Find the unknown dimension.
3. Perimeter = 36 feet
ANSWER
The length of the rectangle is 10 ft.

11. GUIDED PRACTICE
for Examples 3, 4 and 5
Area = 32 mm2 4.
ANSWER
Height of the triangle is 8 mm.

12. GUIDED PRACTICE
for Examples 3, 4 and 5
Area = 6 in.2
ANSWER
Base of the triangle is 3 in.

13. GUIDED PRACTICE
for Examples 3, 4 and 5
6. What If?
In Example 5, suppose the length of the exhibit area was 64 feet instead of 56 feet. Find the area of the region with rocks.
ANSWER
The area of the region with rock is 1920 ft2.