EXAMPLE 3 Standardized Test Practice

A = lw

63 = 9w

63 = 93

9 97 = w

Write area formula.

Substitute values.

Divide each side by 9.

Simplify.

ANSWER The width is 7 feet. The correct answer is A.

SOLUTION

EXAMPLE 4 Finding an Unknown Length

p = 2l + 2w

20 = 2l + 2 (4)

20 – 8 = 2l + 8 – 8

Write perimeter formula.

Substitute 20 for P and 4 for w.

Subtract 8 from each side.

Multiply.

The rectangle at the right has a perimeter of 20 inches. Find the length of the rectangle.

20 = 2l + 8

12 = 2l Simplify.

EXAMPLE 4 Finding an Unknown Length

ANSWER

The length of the rectangle is 6 inches.

2l22

12 =

6 = l

Divide each side by 2.

Simplify.

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.

EXAMPLE 5 Using an Area Formula

40 56 = 20 32 + r

2240 – 640 = 640 – 640 + r

2240 = 640 + r

1600 = r

Write an algebraic model.

Subtract 640 from each side.

Simplify.

The area of the region with rocks is 1600 square feet.

ANSWER

GUIDED PRACTICE for Examples 3, 4 and 5

3. Perimeter = 36 feet

Find the unknown dimension.

ANSWER

The length of the rectangle is 10 ft.

GUIDED PRACTICE for Examples 3, 4 and 5

ANSWER Height of the triangle is 8 mm.

Area = 32 mm24.

GUIDED PRACTICE for Examples 3, 4 and 5

ANSWER Base of the triangle is 3 in.

5. Area = 6 in.2

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.