Radius, Diameter, and Chord

🎯 What Will We Learn Today?

Circles have special lines that help us measure them. Today we'll learn about the radius, diameter, and chord — three important lines inside every circle.

📚 Three Important Lines

The Radius

The radius is a straight line from the center of the circle to any point on the circle's edge. The word "radius" means "ray" in Latin — it shoots out from the center like a sunbeam!

Think of radius as a spoke on a bicycle wheel. Every spoke goes from the center to the edge.

The Diameter

The diameter is a straight line that goes from one side of the circle to the other, passing through the center. It's the longest straight line you can draw inside a circle.

The diameter is exactly twice the radius:

Diameter = 2 × Radius

The Chord

A chord is any straight line connecting two points on a circle. The diameter is a special chord — it's the longest chord because it passes through the center!

🎮 Visualize It!

Imagine a pizza:

The radius is like a cut from the center to the crust

The diameter is like a cut all the way across, through the center

A chord is like any straight cut across the pizza (not going through the center)

💡 Key Formulas

Diameter = 2 × radius (d = 2r)

Radius = diameter ÷ 2 (r = d ÷ 2)

📝 Check Your Understanding

If the radius of a circle is 5 cm, what is its diameter?

What is the longest chord in a circle called?

How many radii equal one diameter?

Draw a circle and label its center, a radius, and a diameter.

Is a radius always longer than a chord? Explain.

🎮 Play a Game!

Practice with circles in Shape Matcher — match shapes and learn geometry!