Circle and Disk. Sphere and Ball
Understanding the difference between boundaries and regions in 2D and 3D
1. What is a Circle (Окружность)?
Definition
A circle is a closed curved line on a plane where all points are at the same distance from one special point called the center.
Key Point: A circle is ONLY the boundary LINE, not the inside!
Remember!
Think of a circle like a hoop or a ring — it's just the edge, with nothing inside!
Interactive visualization — available in the full lesson
2. What is a Disk (Круг)?
Interactive visualization — available in the full lesson
Definition
A disk is the part of a plane bounded by a circle. It includes:
- All points INSIDE the circle
- The circle itself (the boundary)
Key Point: A disk is a REGION (has area), not just a line!
Remember!
Think of a disk like a coin or a pizza — it includes everything inside!