7.3: Similar Polygons and Scale Factors
What if you were comparing a baseball diamond and a softball diamond? A baseball diamond is a square with 90 foot sides. A softball diamond is a square with 60 foot sides. Are the two diamonds similar? If so, what is the scale factor? After completing this Concept, you'll be able to use your knowledge of similar polygons to answer these questions.
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CK12 Foundation: Chapter7SimilarPolygonsandScaleFactorsA
Guidance
Similar polygons are two polygons with the same shape, but not necessarily the same size. Similar polygons have corresponding angles that are congruent, and corresponding sides that are proportional.
These polygons are not similar:
Think about similar polygons as enlarging or shrinking the same shape. The symbol
Example A
Suppose
Just like in a congruence statement, the congruent angles line up within the similarity statement. So,
Example B
In the similarity statement,
Example C
Line up the corresponding sides,
Watch this video for help with the Examples above.
CK12 Foundation: Chapter7SimilarPolygonsandScaleFactorsB
Concept Problem Revisited
All of the sides in the baseball diamond are 90 feet long and 60 feet long in the softball diamond. This means all the sides are in a
Vocabulary
Similar polygons are two polygons with the same shape, but not necessarily the same size. The corresponding angles of similar polygons are congruent (exactly the same) and the corresponding sides are proportional (in the same ratio). In similar polygons, the ratio of one side of a polygon to the corresponding side of the other is called the scale factor.
Guided Practice
1.
2. What is the scale factor of
3.
Answers:
1. All the corresponding angles are congruent because the shapes are rectangles.
Let’s see if the sides are proportional.
2. All the sides are in the same ratio. Pick the two largest (or smallest) sides to find the ratio.
For the similarity statement, line up the proportional sides.
3. From the similarity statement,
Practice
Determine if the following statements are true or false.
 All equilateral triangles are similar.
 All isosceles triangles are similar.
 All rectangles are similar.
 All rhombuses are similar.
 All squares are similar.
 All congruent polygons are similar.
 All similar polygons are congruent.
 All regular pentagons are similar.

△BIG∼△HAT . List the congruent angles and proportions for the sides.  If
BI=9 and \begin{align*}HA = 15\end{align*}, find the scale factor.  If \begin{align*}BG = 21\end{align*}, find \begin{align*}HT\end{align*}.
 If \begin{align*}AT = 45\end{align*}, find \begin{align*}IG\end{align*}.
 Find the perimeter of \begin{align*}\triangle BIG\end{align*} and \begin{align*}\triangle HAT\end{align*}. What is the ratio of the perimeters?
Use the picture to the right to answer questions 1418.
 Find \begin{align*}m \angle E\end{align*} and \begin{align*}m \angle Q\end{align*}.
 \begin{align*}ABCDE \sim QLMNP\end{align*}, find the scale factor.
 Find \begin{align*}BC\end{align*}.
 Find \begin{align*}CD\end{align*}.
 Find \begin{align*}NP\end{align*}.
Determine if the following triangles and quadrilaterals are similar. If they are, write the similarity statement.
 \begin{align*}\triangle ABC \sim \triangle DEF\end{align*} \begin{align*}{\;}\end{align*} Solve for \begin{align*}x\end{align*} and \begin{align*}y\end{align*}.
 \begin{align*}QUAD \sim KENT\end{align*} \begin{align*}{\;}\end{align*} Find the perimeter of \begin{align*}QUAD\end{align*}.
 \begin{align*}\triangle CAT \sim \triangle DOG\end{align*} \begin{align*}{\;}\end{align*} Solve for \begin{align*}x\end{align*} and \begin{align*}y\end{align*}.
 \begin{align*}PENTA \sim FIVER\end{align*} \begin{align*}{\;}\end{align*} Solve for \begin{align*}x\end{align*}.
 \begin{align*}\triangle MNO \sim \triangle XNY\end{align*} \begin{align*}{\;}\end{align*} Solve for \begin{align*}a\end{align*} and \begin{align*}b\end{align*}.
 Trapezoids \begin{align*}HAVE \sim KNOT\end{align*} Solve for \begin{align*}x\end{align*} and \begin{align*}y\end{align*}.
 Two similar octagons have a scale factor of \begin{align*}\frac{9}{11}\end{align*}. If the perimeter of the smaller octagon is 99 meters, what is the perimeter of the larger octagon?
 Two right triangles are similar. The legs of one of the triangles are 5 and 12. The second right triangle has a hypotenuse of length 39. What is the scale factor between the two triangles?
 What is the area of the smaller triangle in problem 30? What is the area of the larger triangle in problem 30? What is the ratio of the areas? How does it compare to the ratio of the lengths (or scale factor)? Recall that the area of a triangle is \begin{align*}A=\frac{1}{2} \ bh\end{align*}.
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Term  Definition 

Congruent  Congruent figures are identical in size, shape and measure. 
Proportion  A proportion is an equation that shows two equivalent ratios. 
Scale Factor  A scale factor is a ratio of the scale to the original or actual dimension written in simplest form. 
sine  The sine of an angle in a right triangle is a value found by dividing the length of the side opposite the given angle by the length of the hypotenuse. 
Trigonometric Ratios  Ratios that help us to understand the relationships between sides and angles of right triangles. 
Image Attributions
Here you'll learn how to identify similar polygons and how to use scale factors to solve for missing sides of similar polygons.