Circles Vocab

Circles Vocab

5.1 Radius, Diameter, and Circumference

Poll

Which are you today?

The Basics

A circle is a set of points equidistant from a given point. The radius of a circle, r, is the distance from the center of the circle to the circle. A circle's size depends only on its radius. The center of the circle below is point A. We call this circle “circle A,” and it is labeled ⨀A.

Open Ended

Why is it important that we labeled this circle ⨀A and not A?

Vocab to Learn

• Radius: The distance from the center of the circle to its outer rim.

• Chord: A line segment whose endpoints are on a circle.

• Diameter: A chord that passes through the center of the circle.

• Secant: A line that intersects a circle in two points.

• Tangent: A line that intersects a circle in exactly one point.

• Point of Tangency: The point where a tangent line touches the circle.

Multiple Choice

What is the name of this circle?

• K

• IJ

• JKI

• KI

Multiple Choice

Which segment is a chord?

• $\overline{AF}$

• $\overline{BH}$

• $\overline{CD}$

Multiple Choice

What is the radius of a circle with a diameter of 45 miles?

• 22.5 miles
• 141.3 miles
• 282.6 miles
• 90 miles

Multiple Choice

What is  $\overleftarrow{B}\overrightarrow{D}$  ?

• a chord

• a secant line

• a tangent line

Let's say as many statements about this diagram as possible.

At least 3 in school and 3 at home participants!

Circumference

Circumference is the distance around a circle. The circumference can also be called the perimeter of a circle. However, we use the term circumference for circles because they are round.

Circumference Formula: C=πd where the diameter d=2r, or twice the radius. So C=2πr as well.

Tips for Calculations:

• For our class, unless specifically noted in a word problem, we will ALWAYS leave our answers “in terms of pi.” This means, you should not approximate your answer in a calculator, rather keep it precise, ie. 72π or 4π. You can write "pi" on this quizizz.

• Remember that you can use formulas in multiple directions... fill in the correct unknown and just solve using algebra!

Multiple Select

What two formulas can you use to find the circumference?

• C = π·d

• C = π·r²

• C = 2·π·r

• C = d²