Mean Geometrically

A and B are two points on a circle centre O. Tangents at A and B cut at C. CO cuts the circle at D. What is the relationship between areas of ADBO, ABO and ACBO?

Just Rolling Round

P is a point on the circumference of a circle radius r which rolls, without slipping, inside a circle of radius 2r. What is the locus of P?

Giant Holly Leaf

Find the perimeter and area of a holly leaf that will not lie flat (it has negative curvature with 'circles' having circumference greater than 2πr).

Pericut

Stage: 4 and 5 Challenge Level:

Thanks are due to Sanjay Joshi of The Perse School, Cambridge for this solution.

 We shall prove that the line $AOB$ always divides the total perimeter into two equal parts, both of length $\pi r$, defining $r$ to be the radius of the large semicircle as in the diagram.

First consider the case where the line $AOB$ is horizontal. The perimeter of the bottom half (the large semicircle) is $\pi r$. Twice the perimeter of each of the small upper semicircles is $2 {\pi r \over 2}$, again $\pi r$. Hence when the line $AOB$ is horizontal, the section of the perimeter above the line and the section below the line are of equal length. As the line $AOB$ rotates about $O$, the lengths of the two sections of the perimeter change to $\pi r + P - Q$ on one side of the line and $\pi r + Q - P$ on the other (where $P$ and $Q$ are lengths as defined in the diagram above). $$Q = r \theta$$ where $\theta$ in this equation is the angle shown in the diagram, measured in radians. Using the property that the angle at the centre of a circle is twice the angle at the circumference subtended by the same arc, $$P = 2\theta ({r \over 2})= r\theta.$$ It is therefore clear that $$P=Q$$ and that the perimeter length will always be $\pi r$.