Examples, solutions, videos, and lessons to help High School students learn how to use
congruence and similarity criteria for triangles to solve problems
and to prove relationships in geometric figures.
Common Core: HSG-SRT.B.5
The following diagrams show the properties of similar triangles. Scroll down the page for more examples and solutions on how to identify similar triangles and how to use similar triangles to solve problems.
Finding missing measures using similar triangles
Use similar triangles to find unknown measures (angles and sides).
Indirect Measurement Using Similar Triangles
This video explains how to use the properties of similar triangles
to determine the height of a tree.
Use Similar Triangles to Solve Problems
This lesson works though three examples of solving problems using
Example 1: Fred needs to know how wide a river is. He takes
measurements as shown in the diagram. Determine the river's width
Example 2: Determine the ratio of the areas of the two similar
Example 3: If the area of the smaller triangle is 20 m2
determine the area of the bigger triangle.
Application of Similar Triangles
An abstract artist wants to create two proportional triangular
paintings. The dimensions are as shown. How long should the two
missing sides be in the second painting?
How Tall Is It (The height of the light pole)
A lesson on using similar triangles and proportions to solve for a
Using Similar Triangles
Examples of applications with similar triangles.
1. A tree with a height of 4 m casts a shadow 15 m long on the
ground. How high is another tree that casts a shadow which is 20 m
2. Jordan wants to measure the width of a river that he can't cross.
Help him to figure out the width of the river.
Indirect Measurement using Similar Triangles
Benjamin places a mirror 40 ft from the base of an oak tree. When he
stands at a distance of 5 ft from the mirror, he can see the top of
the tree in the reflection. If Benjamin is 5 ft 8 in tall, what is
the height of the oak tree?
Word Problems with Similar Triangles and Proportions
1. Two ladders are leaning against a wall at the same angle. How long
is the shorter angle?
2. Campsites R and S are on opposite sides of a lake. What is the
distance between the two campsites?
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