Lesson 8.1 Circle and Circumference

Lesson 8.1 Circle and Circumference

Circle

A set of all points in a plane that are the same distance from the center point.

Diameter

The distance across the circle through the center.

Diameter d = 2r

Radius

The radius is the distance from the center to any point on the circle.

r (radius)  =  $\frac{d}{2}$   

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Circumference

The distance around a circle. The ratio  $\frac{circumference}{diameter\ }$  is the same for EVERY circle and is represented by the Greek letter  $\pi$  , called pi.  The value of  $\pi$   can be approximated as 3.14 or  $\frac{22}{7}$  .

                      C =  $\pi$  d              or                  C = 2 $\pi$  r

When to use decimal or fraction for pi

When the radius or diameter is a multiple of 7, it is easier to use $\frac{22}{7}$   as the estimate of  $\pi$  .  Be sure to read the directions to see what they would like for you to use. Do NOT round until the final answer if they ask you to round.

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Semicircle Perimeter

The distance around the curved part (one-half the circumference of a circle) plus the diameter. 

Simply use the same formula for circumference and the divide by 2. Do not forget to add the diameter length to this.

Semicircle Perimeter =  $\frac{\pi d}{2}+\ d\$       or        $\pi r\ +\ 2r\$  

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