McGraw Hill Integrated II, 2012
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McGraw Hill Integrated II, 2012 View details
3. Geometric Probability
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Exercise 53 Page 905

The area of the shaded region is the area of the circle minus the area of the triangle. To find the area of the triangle, draw a height and find a 30^(∘)-60^(∘)-90^(∘) triangle.

66.3 square centimeters

Practice makes perfect

We are given a diagram and are asked to find the area of the shaded region. Let's take a look at the diagram.

To find the area of the shaded region, we will subtract the area of the triangle from the area of the circle.

Finding the Area of the Triangle

Let's start by finding the area A_T of the triangle. Notice that all the sides of the triangle are congruent, so the triangle is equilateral. This means that every angle in the triangle has a measure of 60^(∘).

As we can see, by drawing the height of the triangle we create a 30^(∘)-60^(∘)-90^(∘) triangle. In a 30^(∘)-60^(∘)-90^(∘) triangle, the longer leg is sqrt(3) times the length of the shorter leg. h = sqrt(3) * 3sqrt(3) ⇔ h = 9 Let's calculate the area A_T of the triangle.

A_T = 1/2 bh
A_T = 1/2 ( 6sqrt(3))( 9)
A_T = 46.765372...
A_T ≈ 46.77

The area of the triangle is approximately 46.77 square centimeters.

Finding the Area of the Circle

Now we will calculate the area A_C of the circle. Let's take a closer look at the circle.

We can see from the diagram that the length of the diameter of our circle is 12 centimeters. We know that the diameter is twice the length r of the radius. 12 = 2 r ⇔ r = 6 Knowing that the radius of our circle is 6 centimeters, we can calculate its area.

A_C = π r^2
A_C = π ( 6)^2
A_C = 113.097336...
A_C ≈ 113.10

The area of the circle is approximately 113.10 square centimeters.

Finding the Area of the Shaded Region

The area A of the shaded region is the area A_C of the circle minus the area A_T of the triangle.

A = A_C - A_T
A ≈ 113.10 - 46.77
A ≈ 66.33
A ≈ 66.3

The area of the shaded region, rounded to the nearest tenth, is 66.3 square centimeters.