In this explainer, we will learn how to use the properties of similar polygons to solve algebraic expressions and equations.
We begin by recapping what it means for two polygons to be similar.
Definition: Similar Polygons
Two polygons are similar if they have the same number of sides, their corresponding angles are congruent, and the lengths of their corresponding sides are proportional.
How To: Expressing the Similarity of Two Polygons and Writing the Similarity Statement
Consider two similar polygons, and , as shown below. The similarity of the two polygons can be written as . The ordering of the letters is important and indicates which vertices of the two polygons correspond to one another. In this example, vertex corresponds to vertex , vertex corresponds to vertex , and so on.
For two similar polygons, the ratio of each pair of corresponding side lengths is the same. This is known as the similarity ratio. For the similar polygons and , the similarity ratio is equal to each of the four ratios below:
We complete our similarity statement for these two polygons by listing the pairs of congruent corresponding angles:
When calculating a similarity ratio, the direction in which we work is important: if the similarity ratio of polygons to is , then the similarity ratio of polygons to is . We must ensure that we always divide the lengths of the same polygon by the corresponding lengths of the other.
In our first example, we recall how to use proportions to calculate an unknown length in a quadrilateral when we can calculate the similarity ratio and the length of the corresponding side is given numerically.
Example 1: Determining an Unknown Length given Similar Quadrilaterals
If is similar to , what is the length of ?
Answer
From the figure, it appears as if the two polygons have been drawn in the same orientation and so, for example, side on the larger polygon corresponds to side on the smaller polygon. This is confirmed by the ordering of the letters in the written statement of similarity: if is similar to , then vertex corresponds to vertex , vertex corresponds to vertex , and so on.
From the figure, we identify that we are given a pair of corresponding lengths, the lengths of sides and . The side whose length we want to calculate, side , corresponds to side on the smaller polygon. As the two polygons are similar, the lengths of their corresponding sides are proportional and so we can write down the similarity ratio for these two pairs of corresponding sides:
By substituting the lengths of , , and , we form an equation:
To solve for , we multiply both sides of the equation by 14 and evaluate:
The length of is 21 cm.
Our first example allowed us to recap the basic principles of how to use the proportionality of corresponding sides in similar polygons to calculate an unknown side length. We will now extend these skills to problems in which at least some of the side lengths are expressed algebraically. We will begin each problem in the same way: using the side lengths we are given, either numerically or algebraically, to set up an equation using the proportionality of pairs of corresponding side lengths. We will then solve this equation to find the unknown value or values.
In our first example of this type, we consider a pair of similar right triangles in which the lengths of two sides of one triangle have been given numerically and the lengths of the corresponding two sides of the other triangle have been expressed algebraically.
Example 2: Forming and Solving an Equation to Find an Unknown Value given Similar Triangles
Given that triangles and are similar, work out the value of .
Answer
We are told that these two triangles are similar and so we begin by identifying pairs of corresponding sides. From the ordering of the letters in the similarity statement, we know that side corresponds to side and side corresponds to side . We have been given numerical values for the lengths of these two sides in triangle and algebraic expressions for the lengths of the corresponding sides in triangle . As the two triangles are similar, corresponding side lengths are proportional and so, using the similarity ratio, we have
Substituting the values and expressions for these four side lengths gives an equation in :
We solve this equation to determine the value of . To eliminate the denominators, we can cross multiply:
Distributing each set of parentheses gives
Finally, we solve for by collecting like terms:
It is sensible to check our answers whenever possible. In the previous example, we could use the value of we found to calculate the lengths of sides and . Substituting into the expressions for each of these side lengths gives
We can then use these lengths to check that corresponding pairs of sides are indeed proportional:
The ratio is the same for both pairs of corresponding sides and so this confirms that our value of is correct.
In each of the two examples we have considered so far, the two similar polygons have been drawn in the same orientation. However, this will not always be the case and great care needs to be taken when approaching any problem to ensure that we check which sides of the two polygons correspond to one another before we begin any calculations. Let us now consider an example in which the two polygons are drawn in different orientations.
Example 3: Forming and Solving an Equation given Similar Pentagons
Given that the two polygons are similar, find the value of .
Answer
We note that the two polygons are clearly drawn in different orientations and so we must first identify which vertices correspond to one another. From the figure, we see that the angle at is congruent to the angle at as both are marked with single arcs. We also observe that the angle at is congruent to the angle at as both are marked with double arcs. We can, therefore, state that , with the ordering of the letters representing which vertices correspond to one another.
The unknown we want to calculate, , forms part of the expressions we have been given for the lengths of sides and . These sides correspond to sides and of the second polygon, the lengths of which are both given. We can, therefore, form an equation using the similarity ratio for these two polygons:
We solve this equation to determine the value of . First, the left-hand side can be simplified by factoring the numerator and then canceling the common factor of 2 in the numerator and denominator. The right-hand side can be simplified by factoring the numerator and then canceling the common factor of 7 in the numerator and denominator:
As 4 is a factor of 12, we can eliminate both denominators by multiplying both sides of the equation by 12 and then canceling common factors:
Distributing the parentheses and collecting like terms gives
Using the similarity ratio for these two similar polygons, we find that .
Some problems may involve more than one unknown variable. In such cases we will need to form and solve more than one equation, but the process will always be the same: we determine the similarity ratio and set up equations using each pair of corresponding side lengths. In our next example, we will use the proportionality of side lengths in a pair of similar polygons to determine two unknowns.
Example 4: Forming and Solving Equations given Similar Quadrilaterals
Given that , find the values of and .
Answer
We note first that the two polygons have been drawn in different orientations. Using the ordering of the letters in the statement of similarity, we can determine which vertices correspond to one another: corresponds to , to , and so on. We may find it helpful to redraw the second polygon in the same orientation as the first, although this is not essential.
We have two unknowns to calculate, and . To begin, we can calculate the similarity ratio using the lengths of the corresponding sides and , both of which have been given numerically:
A similarity ratio of 2 means that the sides of polygon are each twice as long as the lengths of the corresponding sides of polygon . To determine the value of , we consider the similarity ratio between sides and . We now know that or, using the fact that the lengths of polygon are twice as long as the corresponding lengths of polygon , we can take a slightly less formal approach and immediately state that .
Substituting and , we obtain
To determine the value of , we consider the similarity ratio for sides and . Using a logical approach, we know that the length of is half the length of , as is a side of the smaller polygon. Substituting the expression for the length of and the value for the length of gives
We solve this equation to determine the value of :
Hence, the solution is , .
All of the problems we have considered so far have been related to calculating an unknown when it is used to express the length of a side. In our next example, we consider instead how we can use the properties of similar polygons to calculate the value of an unknown when it forms part of the expression for the measure of an angle. This will require a different property of similar polygons: rather than the proportionality of corresponding side lengths, we will use the congruence of corresponding angles.
Example 5: Forming and Solving Equations to Find an Unknown Angle Measure given Similar Quadrilaterals
Given that is similar to , find the values of and .
Answer
We recall first that corresponding angles in similar polygons are congruent. We therefore need to determine which angles in the two quadrilaterals are corresponding. Upon closer inspection of the figure, it is apparent that the two quadrilaterals have not been drawn in the same orientation, as the angles at the top of each are not of equal measure.
Given the order of the letters in the similarity statement, we deduce that
- vertex corresponds to vertex ,
- vertex corresponds to vertex ,
- vertex corresponds to vertex ,
- vertex corresponds to vertex .
We may find it helpful to use colors to indicate corresponding angles in the two polygons, as shown below.
We can now form some equations by equating the expressions or values of the measures of corresponding angles. Considering angles and , we have
To solve for , we subtract 65 from each side of the equation and then divide by 3:
Next, equating the value for angle with the expression for the measure of angle gives
Subtracting 35 from each side of the equation gives
Thus, using the congruence of corresponding angles in similar polygons, we find that and .
We can check our answer to the previous problem by calculating the measures of each angle and confirming that the angle sum in each quadrilateral is indeed . Using , the measure of angle is
The measure of angle is the same as the measure of angle , which is . Summing the four angles in gives which confirms that our value of is correct.
In , the measure of angle is
The measure of angle is the same as the measure of angle , which is . Summing the four angles in gives which confirms that our value of is also correct.
In our final problem, we will consider how we can use the proportionality of side lengths in similar polygons to calculate the perimeter of a triangle in which some of the side lengths are expressed algebraically.
Example 6: Forming and Solving Equations to Find an Unknown Perimeter given Similar Triangles
Given that , determine the perimeter of .
Answer
To determine the perimeter of triangle , we first need to calculate the lengths of each of its three sides. We are given the length of side , but we do not know either of the others. We do know, however, that the two triangles in the figure are similar and so we can express the similarity ratio between the two triangles using corresponding pairs of sides:
Using the expressions given for the lengths of sides and and the numeric values for the lengths of sides and , we can form the equation
We solve this equation to determine . Multiplying both sides of the equation by will eliminate both denominators simultaneously:
Distributing the parentheses and subtracting 10 from each side of the equation gives
This is a quadratic equation in , which can be solved by factoring:
As represents the length of a side, its value must be positive, and so the correct value is .
We now know the lengths of two sides of triangle and need to calculate the third.
Using the similarity ratio for sides , , , and , we have
Substituting , , and gives
We solve it by first multiplying both sides of the equation by :
Next, we divide both sides of the equation by and simplify:
Finally, we calculate the perimeter of triangle by summing its three side lengths:
The perimeter of triangle is 7.2 units.
Let us finish by recapping some of the key points from this explainer.
Key Points
- Two polygons are similar if they have the same number of sides, their corresponding angles are congruent, and the lengths of their corresponding sides are proportional.
- The similarity ratio for a pair of similar polygons is the ratio found by dividing any side length in one polygon by the length of the corresponding side in the other, and it is the same for all pairs of corresponding sides.
- The proportionality of side lengths of similar polygons can be used to find unknown variables when the side lengths have been expressed algebraically. This will require us to form and solve equations using the expressions and values given for each side length.
- As corresponding angles in similar polygons are congruent, unknown values used to express angle measures can be calculated by forming and solving equations in which we first equate the expressions for corresponding angles.
- These skills can be applied to solve problems involving the geometry of similar polygons, such as calculating their perimeters.
