# 6 – Circles Without Coordinates

• Wheels, clocks, gears, rings. Circles are everywhere. They keep our world moving. Not surprisingly, circles are an important topic in geomtery.
• Is this can of tennis balls wider than it is tall? The answer lies in the parts of the circle. In this unit, you’ll learn them all: diameter, radius, circumference.
• What gives you more pizza for the money, two smalls, or one large?
• Once you know how to calculate the area of a circle, you can find the best value for your pizza money.
• Imagine a highway that completely encircles the world. Suppose the builders of that highway need to raise the entire road 50 feet so that boats can pass under it. How much longer will the road be after it’s been raised?
• You’ll solve this with the properties of circles.
• Some parts of a circle are more exciting than others. You’ll learn about the properties of a part of a circle, called an arc, and learn to measure the angle.
• You’ll spend time inside the circle, looking at inscribed triangles, and polygons. You’ll also think outside the circle, by studying secants and tangents. A unicyclist on a wire — that’s a tangent. Acoustic guitars — secant. These ideas are everywhere.
• You’re going to learn what’s great about pi. You might decide to celebrate pi day, by ordering pie, in degrees, or radians.
• Hang on, you’re about to be immersed in circles!