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Getting ready for similarity

Practicing identifying proportional relationships and solving equations with proportions helps us get ready to learn about similarity.
Let’s refresh some concepts that will come in handy as you start the similarity unit of the high school geometry course. You’ll see a summary of each concept, along with a sample item, links for more practice, and some info about why you will need the concept for the unit ahead.
This article only includes concepts from earlier courses. There are also concepts within this high school geometry course that are important to understanding similarity. If you have not yet mastered the Congruent triangles lesson or the Dilations preserved properties lesson, it may be helpful for you to review those before going farther into the unit ahead.

Identifying proportional relationships

What is this, and why do we need it?

A relationship between two quantities is proportional if the ratio between those quantities is always equivalent. We will look at side length ratios to find out whether triangles are similar or not.


Problem 1
Triangle A has a height of 2.5 cm and a base of 1.6 cm. The height and base of triangle B are proportional to the height and base of triangle A.
Which of the following could be the height and base of triangle B?
Choose 3 answers:

For more practice, go to Proportional relationships.

Where will we use this?

Here are a few of the exercises where reviewing proportional relationships might be helpful:

Solve equations with proportions

What is this, and why do we need it?

When two ratios are equal, we create a proportion equation. If we multiply the equation by both denominators, we can solve the resulting equation just like a linear (or quadratic, but not in this unit) equation. We will set up equations with proportions to find lengths in similar figures.


Problem 2.1
Solve for m.
Do not round. If needed, write your answer as a fraction.
  • Your answer should be
  • an integer, like 6
  • a simplified proper fraction, like 3/5
  • a simplified improper fraction, like 7/4
  • a mixed number, like 1 3/4
  • an exact decimal, like 0.75
  • a multiple of pi, like 12 pi or 2/3 pi

For more practice, go to Solving proportions 2.

Where will we use this?

Here are a few exercises where reviewing proportions equations might be helpful.

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