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Question

A square with sides of length 6 cm is given. The boundary of the shaded region is defined by two semi-circles whose diameters are the sides of the square, as shown.
The area of the shaded region is __________ $cm^2$.

The correct answer is
18

1. Identify the Given Information

  • The square has a side length of \( s = 6 \) cm.
  • Two semi-circles are drawn inside the square. Their diameters are the left and bottom sides of the square.
  • The radius of each semi-circle is half the side of the square: $ r = \frac{6}{2} = 3 \text{ cm} $

2. Analyze the Shaded Region

By observing the image, we can see that the shaded region consists of two identical semi-circles, but the area where they overlap (the "leaf" shape in the bottom-left corner) is unshaded (white). Thus, the total shaded area is the sum of the areas of the two semi-circles minus twice the area of their overlap.

Let's divide the \( 6 \times 6 \) square into four smaller \( 3 \times 3 \) squares. The intersection of the two semi-circles happens entirely within the bottom-left \( 3 \times 3 \) square.

3. Mathematical Calculation

  • Area of one semi-circle: $ A_{\text{semi}} = \frac{1}{2} \pi r^2 = \frac{1}{2} \pi (3)^2 = 4.5\pi \text{ cm}^2 $
  • Area of the overlap (Leaf shape): The overlap is formed by two quarter-circles inside a \( 3 \times 3 \) square. The area of one quarter-circle is \( A_{\text{qc}} = \frac{1}{4} \pi (3)^2 = 2.25\pi \). The area of the overlap is: $ A_{\text{overlap}} = 2 \times A_{\text{qc}} - \text{Area of } 3\times3 \text{ square} $ $ A_{\text{overlap}} = 2(2.25\pi) - 3^2 = 4.5\pi - 9 $
  • Total Shaded Area: According to the visual, the shaded area is the union of the two semi-circles, excluding the overlap twice (since it's white in both): $ \text{Area}_{\text{shaded}} = A_{\text{semi 1}} + A_{\text{semi 2}} - 2 \times A_{\text{overlap}} $ Substituting the values: $ \text{Area}_{\text{shaded}} = 4.5\pi + 4.5\pi - 2(4.5\pi - 9) $ $ \text{Area}_{\text{shaded}} = 9\pi - (9\pi - 18) $ $ \text{Area}_{\text{shaded}} = 18 \text{ cm}^2 $
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Important Questions from Mensuration and Geometry

  1. In the given figure, PQRS is a square of side 2 cm and PLMN is a rectangle. The corner L of the rectangle is on the side QR. Side MN of the rectangle passes through the corner S of the square.
    What is the area (in cm²) of the rectangle PLMN?
    Note: The figure shown is representative.

  2. A regular dodecagon (12-sided regular polygon) is inscribed in a circle of radius $r$ cm as shown in the figure. The side of the dodecagon is $d$ cm. All the triangles (numbered 1 to 12) in the figure are used to form squares of side $r$ cm and each numbered triangle is used only once to form a square.
    The number of squares that can be formed and the number of triangles required to form each square, respectively, are:
    Note: The figure shown is representative.

  3. Which one of the following options has the correct sequence of objects arranged in the increasing number of mirror lines (lines of symmetry)?
  4. A circle with center at $(x, y) = (0.5, 0)$ and radius $= 0.5$ intersects with another circle with center at $(x, y) = (1, 1)$ and radius $= 1$ at two points. One of the points of intersection $(x, y)$ is:
  5. During a half-moon phase, the Earth-Moon-Sun form a right triangle. If the Moon-Earth-Sun angle at this half-moon phase is measured to be $89.85^{\circ}$, the ratio of the Earth-Sun and Earth-Moon distances is closest to
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