# Areas of Parallelograms and Triangles

While you may not see the similarities between

parallelograms and

triangles initially, we will come to see that they are actually quite

related when it comes to area.

But, just how similar can they be if one is a three-sided polygon, and the other

is a specific type of quadrilateral? Let’s begin drawing some connections between

parallelograms and triangles by finding out how to measure the areas of parallelograms

first.

## Areas of Parallelograms

Recall that parallelograms are special a type of

quadrilateral whose opposite sides do not intersect. Parallelograms come

in the form of

rectangles, rhombuses, and squares. When trying to determine the area of

a parallelogram, it will be necessary to identify two main components: the **base**

and the **height** of the parallelogram. The base of a parallelogram can be on

any side of the figure. The height of a parallelogram is the perpendicular distance

between any two parallel bases. Let’s look at these different parts in the figure

below.

*We could have also chosen the left and right sides of the parallelogram as our bases.
In this case, the height would run horizontally.*

The area of a parallelogram is given by the product of the base and the height.

That is, areas of parallelograms can be expressed as

where ** A** represents the area,

**is the base, and**

*b*

*h*is the height.

Let’s practice using this formula by doing the exercise below.

### Exercise 1

**Find the area of parallelogram ABCD.**

**Answer:**

First, we want to choose a side as the base of our parallelogram. In this case,

we can choose the side labeled *12***inches** as our base.

The height of our parallelogram is a segment perpendicular to the base we have chosen.

In the diagram, we see that there is a right angle at the intersection of *DC*

and the dotted, blue line with length of *8***inches**. Now that

we have the measures of our base and height, we can plug these values directly into

our area formula. So, we have

The area of parallelogram ** ABCD** is

*96***square inches**.

That means we could fit

**one inch by one inch squares perfectly**

*96*into the parallelogram.

Let’s try one more example to make sure we understand how to use the area formula

for parallelograms.

### Exercise 2

**If the area of parallelogram EFGH is 112 square meters, what must the**

value of x be?

**Answer:**

We cannot simply “plug and chug” for this exercise as we did in the first example.

In this exercise, we are given the area of the parallelogram and must work backwards.

We are given that the height of the parallelogram is ** x** meters. This

is the variable we will solve for. We are also given that the base of our parallelogram,

**, is split up into two smaller lengths: one that is**

*HG*

*11***meters**long and another that is

*5***meters**long. Let’s

combine these to find out what our base is:

So, we’ve found that our base, ** b**, is

**long.**

*16*metersLet’s plug what we know into our area formula and solve for

**.**

*x*

Thus, the height of parallelogram ** EFGH** is

**.**

*7*meters(Notice that we did not say the height was

**because**

*7*meters squaredwe are not talking about area; we are talking about the height.)

Now, let’s examine the area properties of triangles in order to establish the connections

they have with parallelograms.

## Areas of Triangles

Similar to parallelograms, the main components of a triangle’s area are the base

and height of the triangle. The base of a triangle can be any side. The height of

a triangle is the length of the triangle’s **altitude** drawn to the chosen base.

The altitude of a triangle is the line a through a vertex which is perpendicular

to the opposite side of the triangle. Let’s examine these parts in the triangle

below.

The area of a triangle is one-half the product of the base and its corresponding

height. Thus, the formula is

Let’s practice using this formula by doing the following exercises.

### Exercise 3

**Find the area of ?KLM.**

**Answer:**

The base of the triangle, which is segment ** ML**, has a length of

**. Therefore, we will be able to plug in**

centimeters

*24*centimeters

**for**

*24***when we want to use our area formula. The length of the altitude,**

*b*or the height, is also given. We see that the height here is

**,**

*7*centimetersso we can plug

**in for**

*7***. We have both components of**

*h*our formula, so we can plug these numbers in to find the area.

So, the area of triangle ** KLM** is

**.**

*84*square centimetersLet’s try one more example.

### Exercise 4

**Find the area of ?NPQ.**

**Answer:**

Like we noticed in the previous exercise, finding the area of the triangle is as

simple as finding the lengths of the base and height. However, for this exercise,

we will have to be careful in order to not mistake the values of our base and height.

Let’s look for our base first. The segment ** NQ** is our base with a length

of

**. However, we see that there is a dotted line extending**

*10*feetfrom

**in the direction of point**

*Q***. This line is not**

*R*part of our triangle. The only purpose it serves it to help us find the height of

our triangle, as we will see.

In this case, our height (and altitude) lies on the outside of our triangle. Recall

that, by definition, an altitude is the segment perpendicular to a base of a triangle

that passes through the opposite point. Since ** NQ** was chosen as our

base, we needed to extend that segment out to point

**in order for**

*R*it to be possible to have a line that is perpendicular to

**that**

*NQ*passes through

**. We see that the height of**

*P***is**

*?NPQ***. Now, we are ready to plug the values of our base and height**

*6*feetinto the area formula for triangles.

So, the area of ** ?NPQ** is

**.**

*30*square feet
Now, that we are familiar with the area formulas of parallelograms and triangles,

let’s figure out what the relationship between them is.

## Relationship Between Parallelograms and Triangles

Looking strictly at their formulas, we notice that the formula for a triangle has

an extra factor of one-half, whereas the area formula for a parallelogram is just

the base multiplied by the height. Let’s figure out how this extra factor of one-half

comes into play.

Triangles are the most basic type of polygon that make up all other types of polygons.

In other words, we can fit a certain number of congruent triangles into any kind

of polygon. More specifically, we learned that ** (n-2)** triangles can

fit into an

**(see**

*n*-gonpolygons). Thus, when we are working with parallelograms, which are a type

of quadrilateral, we know that we can fit

**congruent triangles**

*(4-2)=2*into the parallelogram.

*It is possible to split up parallelogram ABCD into two congruent triangles: ?ABD
and ?CDB.*

Therefore, if we have a parallelogram of the same base and height as a triangle,

we know we can put two of these triangles together to create the parallelogram.

Let’s take a look at this by using the diagrams below.