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# How to Find the Area of the Shaded Region Using the Area Decomposition Method

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Ray is a licensed engineer in the Philippines. He loves to write about mathematics and civil engineering.

How do you find the area of the shaded region? It has been a constant struggle for students taking up geometry classes on how to solve the shaded areas. The best way to find a shaded region area is to familiarize yourself with the area decomposition method. The Area decomposition method separates the basic shapes found in a given complex figure for fast and organized analysis. Two cases are present in obtaining the areas of the shaded regions - figures with holes and composite figures.

### Case 1: Area of Composite Figures

A composite plane figure is a plane figure made up of different geometric figures whose areas can be determined. The total area of a composite plane figure is equal to the sum of the areas of its parts. If the given shape is a composite figure, subdivide it into different fundamental shapes or polygons of known area formulas. See the step-by-step procedure on how to compute the area of complex figures.

• Determine the basic shapes represented in the problem. For composite shapes, subdivide the whole figure into forms. Once you identify the basic figures present, remember the area equations for each.
• Calculate the areas of all shapes identified.
• Find the area of the shaded region by adding all obtained area values of the different primary shapes to complete the whole figure. Use the formula shown:

Area of the shaded region = Area of Region 1 + Area of Region 2 + Area of nth Region

### Case 2: Outer Area Minus Inner Area

For figures containing holes or cavities, consider the whole figure's area without the hole or cavity. The remaining area is obtained by subtracting the punch hole area from the entire figure. Use the step-by-step procedure shown below in solving for areas of shaded regions with holes.

• Determine the basic shapes represented in the problem. Once you identify the basic figures present, remember the area equations for each figure.
• Calculate the areas of both inner and outer shapes.
• Find the area of the shaded region by subtracting the size area of the unshaded inner shape from the outer, larger shape. The area outside the inner figure is the part that indicates the area of interest. Use the formula shown:

Area of the shaded region = Area of the outer shape - Area of the unshaded inner shape

## Example 1: Finding the Area of the Inscribed Four-Leaf Figure

If the area of the square shown in the figure below is 64 square inches. Find the area value of the inscribed four-leaf figure.

Solution

There are so many ways to solve this inscribed four-leaf figure problem. You can use simple geometrical analysis or a calculus method. However, this time, let us focus on the simple geometrical analysis. First and foremost, obtain the length of the square sides given its area of 64 square inches.

Asquare = s2
Asquare = 64 square inches
s2 = 64
s = √64
s = 8

As you can observe from the photo shown, the curves from the four-leaf figure resemble that of a half-circle. Also, as we draw these half-circles running through the lines of the four-leaf, some areas are excluded. These areas are the unshaded regions we need to solve for the four-leaf area.

Aunshaded region 1 = Asquare - Asemicircle #1 - Asemicircle #2
Aunshaded region 1 = 82 - π(4)2/2 - π(4)2/2
Aunshaded region 1 = 64 - 16π
Aunshaded region 1 = 13.73 square inches

Aunshaded region 2 = Asquare - Asemicircle #3 - Asemicircle #4
Aunshaded region 2 = 82 - π(4)2/2 - π(4)2/2
Aunshaded region 2 = 64 - 16π
Aunshaded region 2 = 13.73 square inches

Finally, solve for the area of the four-leaf figure.

Afour-leaf figure = Asquare - Unshaded area
Afour-leaf figure = 64 - 13.73 - 13.73
Afour-leaf figure = 36.54 square inches

The area of the inscribed four-leaf figure is 36.54 square inches.

## Example 2: Finding the Area of a Composite Figure Composed of a Triangle, Rectangle, and Semicircle

A composite figure consists of a triangle, rectangle, and semicircle. The triangle's altitude is three times the radius of the semicircle. The height of the rectangle is four times the radius of the semicircle. If the area of the composite figure is 201 square centimeters, find the radius of the semicircle.

Solution

For the given composite figure, it shows that all are shaded shapes. To solve this problem, directly add all areas of the triangle, rectangle, and semicircle. Let r be the radius of the semicircle. Given all regions are expressed in terms of r, solve for the value of r.

Atriangle = ½ x b x h
Atriangle = ½ (2r) (3r)
Atriangle = 3r2

Arectangle = l x w
Arectangle = (4r) (2r)
Arectangle = 8r2

Asemicircle = π(r)2/2

Solve for the total area of the given complex shape by adding all area measurements obtained for triangle, rectangle, and semicircle.

Total Area = Atriangle + Arectangle + Asemicircle
201 = 3r2 + 8r2 + (π/2)r2
201 = 11r2 + (π/2)r2
r = 4 centimeters

The radius of the semicircle is 4 centimeters.

## Example 3: Area of the Shaded Region in a Right Triangle

Solve for the area of the shaded region in the right triangle shown below.

Solution

It is a simple problem of finding the area of shaded regions on right triangles. Subtract the area value of the unshaded inner shape from the outer shape area to get the area measurement of the shaded region.

Aouter shape = ½ x b x h

Aouter shape = ½ (10) (8)

Aouter shape = 40 square inches

Ainner shape = ½ x b x h

Ainner shape = ½ (8) (6)

Ainner shape = 24 square inches

Finally, find the area of the shaded region.

Ashaded region = Area of the outer shape - Area of the unshaded inner shape
Ashaded region = 40 - 24
Ashaded region = 16 square inches

The area of the shaded region is equal to 16 square inches.

## Example 4: Area of Unshaded Region in a Right Triangle

Given the dimensions of the right triangle shown, solve for the area of the unshaded region.

Solution

Treat the triangles as similar triangles. Then, subtract the area of the shaded region from the larger right triangle.

Abigger triangle = ½ x b x h

Abigger triangle = ½ (48 + 42) (74)

Abigger triangle = 3330 square centimeters

Asmaller triangle = ½ x b x h

Asmaller triangle = ½ x (42) (48)

Asmaller triangle = 1008 square centimeters

Find the area of the unshaded region by subtracting the smaller triangle's area value from that of the larger triangle.

Aunshaded region = Abigger triangle - Asmaller triangle
Aunshaded region = 3330 - 1008
Aunshaded region = 2322 square centimeters

The area of the unshaded region in the right triangle is 2322 square centimeters.

## Example 5: Finding the Area of the Shaded Region of a Rectangle

Calculate the area of the shaded region of the rectangle shown below.

Solution

This problem is an example of case 2. Subtract the punch hole area, the inner rectangle, from the whole figure of dimensions 15 and 30 centimeters.

Alarger rectangle = l x w

Alarger rectangle = 30 x 15

Alarger rectangle = 450 square centimeters

Ainner rectangle = l x w

Ainner rectangle = 12 x 24

Ainner rectangle = 288 square centimeters

Ashaded region = Alarger rectangle - Ainner rectangle

Ashaded region = 450 - 288

Ashaded region = 162 square centimeters

The area of the shaded region in the rectangle is 162 square centimeters.

## Example 6: Area of the Shaded Region of Square Diamond

Given that the larger square side is equal to 6 m, solve for the shaded area's value, the side length of the diamond square inscribed is 5 meters.

Solution

Find the area of the larger square using the formula s2.

Alarger square = s2

Alarger square = 6 x 6

Alarger square = 36 square meters

Compute for the area value of the smaller square.

Asmaller square = s2
Asmaller square = 5 x 5
Asmaller square = 25 square meters

Find the area of the shaded region by subtracting the obtained area of the smaller square from the area measurement of the larger square shape.

Ashaded region = Abigger square - Asmaller square
Ashaded region = 36 - 25
Ashaded region = 11 square meters

The area of the shaded region is equal to 11 square meters.

## Example 7: Area of the Shaded Region in Between an Inscribed Triangle in a Circle

The diameter of the larger circle is 25 centimeters. If the triangle inside the circular shape is an equilateral triangle with a side length of 15 centimeters, what is the area of the shaded region?

Solution

Solve for the value of the larger circle given the diameter of 25 centimeters. Use the formula A = πD2 / 4.

Abigger circle = πD2 / 4

Abigger circle = π(25)2 / 4

Abigger circle = 625π/4 square centimeters

Abigger circle = 490.87 square centimeters

Find the area value of the inner triangle using the formula shown below. There are many formulas to solve equilateral triangles' areas, such as heron's formula or the Pythagorean theorem. It is entirely up to you what to use.

Ainner triangle = √3/4 (a)2
Ainner triangle = √3/4 (15)2
Ainner triangle = 97.43 square centimeters

Finally, solve for the area measurement of the shaded region.

Ashaded region = Abigger circle - Ainner triangle
Ashaded region = 490.87 - 97.43

The area of the shaded region is equal to 390.44 square cm.

## Example 8: Calculating the Area of the Shaded Region in a Composite Shape

Calculate the shaded area of the complex figure with a circular hole in it, given that the diameter of the hole is 24 inches.

First, compute the area of the whole composite shape given the dimensions shown in the figure.

Acomposite figure = (60 x 25) + (25 x 15)
Acomposite figure = 1875 square inches

Then, compute the area of the inner circle, given that its diameter is 24 inches.

Ainner circle = πD2 / 4
Ainner circle = π(24)2 / 4
Ainner circle = 144π
Ainner circle = 452.39 square inches

Calculate the area of the shaded region by subtracting the area value of the inner circle from the area value of the composite shape.

Ashaded region = Acomposite figure - Ainner circle
Ashaded region = 1875 - 452.39 square inches

The shaded area in the complex figure given is 452.39 square inches.

## Example 9: Finding the Area of the Shaded Region

If triangle ABC is a right triangle, and BC is a semicircle, calculate the area of the shaded diagram below.

Solution

The total area is the sum of the areas of the semicircle BC and the right triangle ABC. Note that the length of the side and altitude of the right triangle are known. Solve for the area using the simple equation ½ (b) (h).

Aright triangle = ½ (b) (h)

Aright triangle = ½ (4) (3)

Aright triangle = 6 square units

Then, solve for the area of the semicircle BC. By observation, the diameter of semicircle BC is the hypotenuse of the right triangle ABC. Use the Pythagorean theorem to solve the hypotenuse length. Then proceed on getting the area of the semicircle. Let c be the hypotenuse.

c2 = a2 + b2

c = a2 + b2

c = (3)2 + (4)2

c = 5

Asemicircle = πD2 / 8

Asemicircle = π(5)2 / 8

Asemicircle = 25π / 8

Asemicircle = 9.82 square units

Finally, solve for the area of the shaded region by adding the area measurement of the right triangle and the area value of the semicircle.

Ashaded region = Aright triangle + Asemicircle
Ashaded region = 6 + 9.82
Ashaded region = 15.82 square units

The total area is equal to 15.82 square units.

## Example 10: Finding the Area of a Complex Shape

If all angles are right angles in the figure below, find the total area of the complex shape using the area decomposition method. Take note that all angles are right angles.

Solution

Divide the given complex figure into six basic shapes. The total area is the sum of the rectangles, squares, and triangle areas. Given the values of areas of the basic shapes, add up all of them to compute the area value of the complex form. Just take note of the divisions made from the whole complex body.

Ashaded region = A1 + A2 + A3 + A4 + A5 + A6

Ashaded region = 16 + 4 + 4 + 4 + 2 + 2.5

Ashaded region = 32.5 square inches

The area of the complex shape is 32.5 square inches.

## Example 11: Circle Inscribed in a Triangle

Find the area of the shaded region given that the radius of the inscribed circle is 16 cm and the side lengths of the triangle are 20 cm, 50 cm, and 75 cm.

Solution

Find the area of the outer triangle utilizing only the side lengths of the triangle. Use Heron's formula to solve the triangle's area value given the sides 20 cm, 50 cm, and 75 cm. But first, solve for the semi-perimeter of the triangle.

s = (a + b + c) / 2

s = (20 + 50 + 69) / 2

s = 69.5

Atriangle = s(s-a)(s-b)(s-c)

Atriangle = 69.5 (69.5 - 20)(69.5 - 50)(69.5 - 69)

Atriangle = 183.15 square centimeters

You can also use the generated formula for the area of the circumscribed triangle given below.

Atriangle = ½ (r) (Perimeter of triangle)
Atriangle = ½ (2.64) (20 + 50 + 69)
Atriangle = 183.15 square centimeters

Then, solve for the area of the inner circle with radius r = 2.64 centimeters.

Ainner circle = πr2
Ainner circle =π(2.64)2
Ainner circle = 21.9 square centimeters

Calculate the area of the shaded region by subtracting the area measurement of the inscribed circle from the larger triangle.

Ashaded region = Atriangle - Ainner circle
Ashaded region = 183.15 - 21.9
Ashaded region = 161.25 square centimeters

The calculated area of the shaded region is 161.25 square centimeters.

## Example 12: Area of the Shaded Region in a Circle

What is the area of the shaded region circle? Assume that the diameter of the larger circle is 30 centimeters and the radius of the punched circular hole is 6.5 centimeters.

Solution

Find the area of the shaded region by subtracting the area value of the smaller inner circle from the area value of the larger circle shape.

Ainner circle = πr2

Ainner circle = π(6.5)2

Ainner circle = 132.73 square centimeters

Aouter circle = πD2 / 4

Aouter circle = π(30)2 / 4

Aouter circle = 225π

Aouter circle = 706.86 square centimeters

Ashaded region = Aouter circle - Ainner circle

Ashaded region = 706.86 - 132.73

Ashaded region = 574.13 square centimeters