Lesson 8.1 Circle and Circumference

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

The distance across the circle through the center.

Diameter d = 2r

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

r (radius)  =  $\frac{d}{2}$   

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  $\frac{22}{7}$  .

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.

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\$