*Editor’s note: This lesson has very limited material supported by CK-12. Please refer to the Geometry Access Reader for additional materials.*

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Learning Objectives

- Use similarity to generalize the results.

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Similarity

We know that *all circles are similar to each other.*

Suppose a circle has a radius of units.

- The
*scale factor* of this circle (with radius ) and the circle with radius 1 is:

.

- You know how a
*scale factor* affects area measures:

If the scale factor is , then the area is times as much.

**Reading Check:**

1. *Explain how we know that all circles are similar to each other.*

*(Hint: Think about similar measurements that circles may have.)*

2. *If the scale factor of a circle is , how does this relate to a circle’s area?*