Calculate circumference and area of circles, and understand radius, diameter, and their relationships.
Circumference (perimeter of a circle): [formula] Area: [formula] Key relationships: d = 2r. pi approx 3.14159...
Example: A circle has radius 7 cm. Find its circumference and area.
Always check if the problem gives radius or diameter. This is the #1 error source on circle problems. If diameter is 10, radius is 5 — use 5 in the formulas!
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