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Areas and Ratios

What is the area of the quadrilateral APOQ? Working on the building blocks will give you some insights that may help you to work it out.

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Six Discs

Six circular discs are packed in different-shaped boxes so that the discs touch their neighbours and the sides of the box. Can you put the boxes in order according to the areas of their bases?

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Given a square ABCD of sides 10 cm, and using the corners as centres, construct four quadrants with radius 10 cm each inside the square. The four arcs intersect at P, Q, R and S. Find the area enclosed by PQRS.

Crescents and Triangles

Stage: 4 Challenge Level: Challenge Level:2 Challenge Level:2

The trick with this question was not to make life difficult by working values out. As a consequence some of you managed much more elegant solutions than others. Here is an example based on that offered by Shu Cao (very well done). Jeongmin Lee, Rebecca Bartram (The Mount School, York) and Andrei Lazanu (School 205, Bucharest) should also be congratulated.

Using Pythagoras` theorem in $ABC$,
$$\begin{eqnarray}a &=& \sqrt{b^2 + c^2} \\ \mbox{Area of semicircle on BC} &=& \pi \frac {a^2}{4}\\ &=& S1\\ \mbox{Area of semicircle on AC} &=& \pi \frac {b^2}{4}\\ &=& S2 \\ \mbox{Area of semicircle on AB} &=& \pi \frac{c^2}{4}\\ &=& S3\\ \mbox{Area of crescents} &=& S2 + S3 + \mbox{Area ABC} - S1\\ &=& \pi \frac {b^2}{4} +\pi \frac {c^2}{4} + \mbox{Area ABC} - \pi \frac {a^2}{4}\\ &=& \pi \frac {b^2}{4} + \pi \frac{c^2}{4} - \pi \frac {a^2}{4} + \mbox{Area ABC}\\ &=& \frac{\pi}{4} \times ( b^2 + c^2 - a^2 ) + \mbox{Area ABC}\\ &=& 0 + \mbox{Area ABC} \\ &=& \mbox{Area ABC} \end{eqnarray}$$

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