Math about Circles

Radius and diameters depend on the circles you draw. For example,

• Small circle: Radius 1.5 cm Diameter 3 cm
• Medium circle: Radius 2.3 cm Diameter 4.6 cm
• Large circle: Radius 3 cm Diameter 6 cm

What is the relationship between the radius and diameter of each circle?

• The diameter is 2 times the radius.

Is that true for any circle? Talk about it: Why do you think so?

• Yes

• Because the diameter goes through the center, it is made up of two radii that start in the center. These radii are the same length. So a diameter has to be twice as long as a radius. This will be true no matter what size the circle is, because you can draw a diameter and two radii that make it up.