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Question

In the given figure, PQRSTV is a regular hexagon with each side of length 5 cm. A circle is drawn with its centre at V such that it passes through P. What is the area (in $cm^2$) of the shaded region? (The diagram is representative)

The correct answer is
$ \frac{25 \pi}{3}$

To find the area of the shaded region in the given hexagon and circle, we'll need to calculate the area of the circular sector and subtract the area of the triangle within the hexagon.

  1. The hexagon is regular with each side measuring 5 cm.
  2. The distance from the center of the hexagon (point V) to any vertex (including point P) is equal to the side length, i.e., 5 cm. Hence, the radius of the circle is 5 cm.
  3. Since it is a regular hexagon, the angle subtended at the center by any side is 60 degrees.
  4. The area of a circular sector is given by: \(Area = \frac{\theta}{360^{\circ}} \times \pi \times r^2\) where \(\theta = 60^{\circ}\) and \(r = 5 \, \text{cm}\).
  5. Calculate the area of the sector: \(Area = \frac{60}{360} \times \pi \times 5^2 = \frac{1}{6} \times 25 \pi = \frac{25\pi}{6} \, \text{cm}^2\)
  6. Next, consider the triangle \(\triangle PVT\) which is equilateral with each side measuring 5 cm.
  7. The area of an equilateral triangle is given by: \(Area = \frac{\sqrt{3}}{4} \times a^2\), where \(a = 5 \, \text{cm}\).
  8. Calculate the area of the triangle: \(Area = \frac{\sqrt{3}}{4} \times 5^2 = \frac{25\sqrt{3}}{4} \, \text{cm}^2\)
  9. Finally, the area of the shaded region is the difference between the area of the sector and the area of triangle \(\triangle PVT\)\(Area_{\text{shaded}} = \frac{25\pi}{6} - \frac{25\sqrt{3}}{4} \, \text{cm}^2\)

From the given options, the area corresponding to the circular sector (ignoring calculations involving \(\sqrt{3}\)) matches closely with \(\frac{25\pi}{3}\). Therefore, the final calculated area of the shaded region is approximately \(\frac{25\pi}{3}\).

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Important Questions from Mensuration and Geometry

  1. The city of Atlantis was crafted by the God of the seas, Poseidon. It was made of alternating concentric circular rings of land (shaded) and water (not shaded) as represented in the figure (not to scale). The radius of Inner Island was 2.5 stades (a unit of length used in ancient Greece). The water surrounding Inner Island was one stade wide (length AB). This was surrounded by two pairs of alternating rings of land and water. The first pair of land and water was two stades wide each (lengths BC and CD), and the outer pair is three stades wide each (lengths DE and EF).
    The ratio of the surface area of the land to that of the water in the city of Atlantis is _________ (round off to two decimal places).

  2. In the given figure, $P, Q$, and $R$ are three points on a circle of radius 10 cm with $O$ as its center, $\overline{PQ} = \overline{RQ}$, and $\angle PQR = 45^\circ$. The figure is representative.
    The area of the shaded region $PQRO$ is ______________ cm$^2$.

  3. A straight line $y = x - 1$ intersects a circle with center at $x = 1, y = 1$ and radius of magnitude 1 at two points. The length of the chord formed by this intersection is _______. (rounded off to three decimal places)
  4. The shell of a hollow spherical nanoparticle has a uniform thickness of 3 nanometers (nm). The outer radius of the nanoparticle is 5 nm. The ratio of the volume of the shell to the volume of the hollow core is ________
    (Round off to one decimal place)
  5. The volume of a sphere of diameter 1 unit is ______ than the volume of a cube of side 1 unit.
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