Perimeter and Area

 

Learning Objective(s)

·         Find the perimeter of a polygon.

·         Find the area of a polygon.

·         Find the area and perimeter of non-standard polygons.

 

 

Perimeter and area are two important and fundamental mathematical topics. They help you to quantify physical space and also provide a foundation for more advanced mathematics found in algebra, trigonometry, and calculus. Perimeter is a measurement of the distance around a shape and area gives us an idea of how much surface the shape covers.

 

Knowledge of area and perimeter is applied practically by people on a daily basis, such as architects, engineers, and graphic designers, and is math that is very much needed by people in general. Understanding how much space you have and learning how to fit shapes together exactly will help you when you paint a room, buy a home, remodel a kitchen, or build a deck.

 

 

The perimeter of a two-dimensional shape is the distance around the shape. You can think of wrapping a string around a triangle. The length of this string would be the perimeter of the triangle. Or walking around the outside of a park, you walk the distance of the park’s perimeter. Some people find it useful to think “peRIMeter” because the edge of an object is its rim and peRIMeter has the word “rim” in it.

 

If the shape is a polygon, then you can add up all the lengths of the sides to find the perimeter. Be careful to make sure that all the lengths are measured in the same units. You measure perimeter in linear units, which is one dimensional. Examples of units of measure for length are inches, centimeters, or feet.

 

 

 

 

This means that a tightly wrapped string running the entire distance around the polygon would measure 22 inches long.

 

 

 

 

Sometimes, you need to use what you know about a polygon in order to find the perimeter. Let’s look at the rectangle in the next example.

 

 

 

 

Notice that the perimeter of a rectangle always has two pairs of equal length sides. In the above example you could have also written P = 2(3) + 2(8) = 6 + 16 = 22 cm. The formula for the perimeter of a rectangle is often written as P = 2l + 2w, where l is the length of the rectangle and w is the width of the rectangle.

 

 

 

The area of a two-dimensional figure describes the amount of surface the shape covers. You measure area in square units of a fixed size. Examples of square units of measure are square inches, square centimeters, or square miles. When finding the area of a polygon, you count how many squares of a certain size will cover the region inside the polygon.

 

Let’s look at a 4 x 4 square.

 

 

You can count that there are 16 squares, so the area is 16 square units. Counting out 16 squares doesn’t take too long, but what about finding the area if this is a larger square or the units are smaller? It could take a long time to count.

 

Fortunately, you can use multiplication. Since there are 4 rows of 4 squares, you can multiply 4 • 4 to get 16 squares! And this can be generalized to a formula for finding the area of a square with any length, s: Area = s • s = s².

 

 

You can write “in²” for square inches and “ft²” for square feet.

 

To help you find the area of the many different categories of polygons, mathematicians have developed formulas. These formulas help you find the measurement more quickly than by simply counting. The formulas you are going to look at are all developed from the understanding that you are counting the number of square units inside the polygon. Let’s look at a rectangle.

 

 

You can count the squares individually, but it is much easier to multiply 3 times 5 to find the number more quickly. And, more generally, the area of any rectangle can be found by multiplying length times width.

 

 

 

 

 

 

It would take 24 squares, each measuring 1 cm on a side, to cover this rectangle.

 

The formula for the area of any parallelogram (remember, a rectangle is a type of parallelogram) is the same as that of a rectangle: Area = l • w. Notice in a rectangle, the length and the width are perpendicular. This should also be true for all parallelograms. Base (b) for the length (of the base), and height (h) for the width of the line perpendicular to the base is often used. So the formula for a parallelogram is generally written, A = b • h.

 

 

 

 

 

Find the area of a parallelogram with a height of 12 feet and a base of 9 feet.   A) 21 ft2 B) 54 ft2 C) 42 ft D) 108 ft2   Show/Hide Answer A) 21 ft2 Incorrect. It looks like you added the dimensions; remember that to find the area, you multiply the base by the height. The correct answer is 108 ft2.   B) 54 ft2 Incorrect. It looks like you multiplied the base by the height and then divided by 2. To find the area of a parallelogram, you multiply the base by the height. The correct answer is 108 ft2.   C) 42 ft Incorrect. It looks like you added 12 + 12 + 9 + 9. This would give you the perimeter of a 12 by 9 rectangle. To find the area of a parallelogram, you multiply the base by the height. The correct answer is 108 ft2.   D) 108 ft2 Correct. The height of the parallelogram is 12 and the base of the parallelogram is 9; the area is 12 times 9, or 108 ft2.

 

 

 

The formula for the area of a triangle can be explained by looking at a right triangle. Look at the image below—a rectangle with the same height and base as the original triangle. The area of the triangle is one half of the rectangle!

 

 

Since the area of two congruent triangles is the same as the area of a rectangle, you can come up with the formula Area =  to find the area of a triangle.

 

When you use the formula for a triangle to find its area, it is important to identify a base and its corresponding height, which is perpendicular to the base.

 

 

 

Now let’s look at the trapezoid. To find the area of a trapezoid, take the average length of the two parallel bases and multiply that length by the height: .

 

An example is provided below. Notice that the height of a trapezoid will always be perpendicular to the bases (just like when you find the height of a parallelogram).

 

 

 

 

Area Formulas   Use the following formulas to find the areas of different shapes.   square:    rectangle:    parallelogram:    triangle:    trapezoid:

 

 

 

Often you need to find the area or perimeter of a shape that is not a standard polygon. Artists and architects, for example, usually deal with complex shapes. However, even complex shapes can be thought of as being composed of smaller, less complicated shapes, like rectangles, trapezoids, and triangles.

 

To find the perimeter of non-standard shapes, you still find the distance around the shape by adding together the length of each side.

 

Finding the area of non-standard shapes is a bit different. You need to create regions within the shape for which you can find the area, and add these areas together. Have a look at how this is done below.

 

 

 

 

You also can use what you know about perimeter and area to help solve problems about situations like buying fencing or paint, or determining how big a rug is needed in the living room. Here’s a fencing example.

 

 

 

 

Find the area of the shape shown below.     A) 11 ft2 B) 18 ft2 C) 20.3 ft D) 262.8 ft2   Show/Hide Answer A) 11 ft2 Correct. This shape is a trapezoid, so you can use the formula  to find the area: .   B) 18 ft2 Incorrect. It looks like you multiplied 2 by 9 to get 18 ft2; this would work if the shape was a rectangle. This shape is a trapezoid, though, so use the formula . The correct answer is 11 ft2.   C) 20.3 ft Incorrect. It looks like you added all the dimensions together. This would give you the perimeter. To find the area of a trapezoid, use the formula . The correct answer is 11 ft2.   D) 262.8 ft2 Incorrect. It looks like you multiplied all of the dimensions together. This shape is a trapezoid, so you use the formula . The correct answer is 11 ft2.

 

 

The perimeter of a two-dimensional shape is the distance around the shape. It is found by adding up all the sides (as long as they are all the same unit). The area of a two-dimensional shape is found by counting the number of squares that cover the shape. Many formulas have been developed to quickly find the area of standard polygons, like triangles and parallelograms.