Maths Olympiad Prep

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Problem 705

AMC 10/12, early questions
Geometry Difficulty 3.8 Find the answer CEMC Pascal · Canada · 2026

On each side of the square shown, a semi-circle is drawn inside
the square. The side length of the square is 1010 and is equal to the diameter of each
semi-circle. The four semi-circles overlap to form the shaded four-petal
flower.

If nn is the closest integer to
the area of the shaded flower, what is the value of nn?

A number or a short expression. Fractions can be typed as 3/2, and spacing doesn't matter.

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Official solution

Label the centre of the square AA, the midpoint of the base BB, and the bottom-right corner of the
square CC. Then ABC\triangle ABC has a right angle at BB and AB=CBAB=CB, as shown.

[[IMAGE0]]

The flower is made up of four "petals", each of which is made up of
two copies of the region formed by taking a sector of the circle of
angle 90°90\degree and removing the
right-isosceles triangle formed by the two radii.

The radius of each circle is 102=5\dfrac{10}{2}=5, so the area of each
sector is π(5)24=25π4\dfrac{\pi(5)^2}{4}=\dfrac{25\pi}{4}. The
area of the triangle being removed is 12(5)2=252\dfrac{1}{2}(5)^2=\dfrac{25}{2}.

Therefore, the area of each petal is $2×(25π4252)=25π502$.\$2\times\left(\dfrac{25\pi}{4}-\dfrac{25}{2}\right) = \dfrac{25\pi-50}{2}\$.

The area of the flower is 44
times the area of each petal, which is 4×(25π502)=2×(25π50)=50π10057.08\begin{align*} 4\times\left(\dfrac{25\pi-50}{2}\right) &= 2\times (25\pi - 50) \\ &= 50\pi-100 \\ &\approx 57.08\end{align*} The integer closest to the area
of the flower is 5757.

Source: CEMC, University of Waterloo, licensed CC-BY-NC-4.0. Statement reproduced verbatim; metadata (topic, difficulty, ordering) added by this project.