Skill: Relate the radius and diameter of a circle

The radius $r$ is the distance from the center of a circle to its edge. The diameter $d$ goes all the way across the circle through its center.

A diameter is made of two equal radii, so:

$$d=2r$$

To find the radius from a diameter, divide by $2$:

$$r=\frac{d}{2}$$

Example: A circle has a radius of $(6\text{ cm})$. What is its diameter?

1. Use the relationship $(d=2r)$.

2. Substitute $(r=6\text{ cm})$:

$$d=2(6)$$

3. Multiply:

$$d=12\text{ cm}$$

The diameter is $(12\text{ cm})$.