Circles and Pi

Circumference and Area of Circles Review

I can use the formulas for circumference and area of circles and semi-circles.

Parts of a Circle

• Circle: a set of all points that are equidistant from another single point called the center.

  
  

• r = radius ; D = diameter

  Diameter = 2 x radius or D = 2r

   radius=Diamater/2 or r=D/2

  
  

• Watch the video https://www.youtube.com/watch?v=cC0fZ_lkFpQ

Multiple Choice

Determine the radius of the penny

• 59.66 mm
• 9.5 mm
• 38 mm
• 119.32 mm

Multiple Choice

Is the red line a radius or diameter of the circle?

• Radius

• Diameter

An Important Question

Circumference and Area

• Circumference = π ×Diameter

  C =πD

  Since D=2xr then C=2πr

     

  Area =π ×radius ×radius

  or A=π x r^2

  A=π x r2

    

• https://www.youtube.com/watch?v=cC0fZ_lkFpQ

Multiple Choice

What is the area of this circle?

• 8322.13
• 1134.11
• 4536.46
• 1267.09

Multiple Choice

What does area mean?

• the distance around a shape
• half the distance around a shape
• the amount of space outside a shape
• the amount of space inside a shape

Multiple Choice

What is the formula for finding the area of a circle?