You may also like

problem icon

Rotating Triangle

What happens to the perimeter of triangle ABC as the two smaller circles change size and roll around inside the bigger circle?

problem icon

Intersecting Circles

Three circles have a maximum of six intersections with each other. What is the maximum number of intersections that a hundred circles could have?

problem icon

Square Pegs

Which is a better fit, a square peg in a round hole or a round peg in a square hole?

A Chordingly

Stage: 3 Challenge Level: Challenge Level:3 Challenge Level:3 Challenge Level:3
Prove that the area of the annulus is ${1\over 4}\pi AB^2$ where $AB$ is a tangent to the inner circle.

annulus with AB drawn