Skill: Solve problems involving radius and diameter

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

They are related by:

$$d=2r$$

So:

• To find the diameter, multiply the radius by $2$.
• To find the radius, divide the diameter by $2$.

Example

A circular table has a diameter of $120\text{ cm}$. What is its radius?

Step 1: Identify what is given and what you need.

• Given: diameter, $d=120\text{ cm}$
• Need: radius, $r$

Step 2: Choose the relationship.

To find the radius from the diameter, use:

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

Step 3: Substitute and calculate.

$$r=\frac{120}{2}=60\text{ cm}$$

Answer: The radius of the table is $\boxed{60\text{ cm}}$. The unit stays in centimeters.