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!

Fill in the Blank

A circle with a radius of 7 cm has a circumference of ____ cm.

Fill in the Blank

The circumference of a circle is 64 pi units. The diameter is ___ units.

Multiple Select

What two formulas can you use to find the circumference?

• C = π·d

• C = π·r²

• C = 2·π·r

• C = d²