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Q13:

A round table cover has six equal designs as shown in Fig. 11.11. If the radius of the cover is 28 cm, find the cost of making the designs at the rate of ` 0.35 per cm$^2$. (Use $\sqrt{3} = 1.7$)

Solution :

Given:

1. A circular table cover with radius $r = 28$ cm.

2. There are six equal designs on the table cover, which are segments of the circle.

3. The rate of making the designs is ₹ $0.35$ per cm$^2$.

4. The value of $\sqrt{3} = 1.7$.

To Find:

The total cost of making the six designs.

r = 28 cm O

Step 1: Determine the central angle of each sector.

Since there are 6 equal designs, the circle is divided into 6 equal sectors. The central angle $\theta$ for each sector is:

$\theta = \frac{360^\circ}{6} = 60^\circ$

Step 2: Calculate the area of one design (segment).

The area of a segment is given by the formula: Area of sector - Area of the triangle formed by the two radii and the chord.

Area of one sector = $\frac{\theta}{360^\circ} \times \pi r^2$

Using $\pi = \frac{22}{7}$ and $r = 28$:

Area of one sector = $\frac{60}{360} \times \frac{22}{7} \times 28 \times 28$

Area of one sector = $\frac{1}{6} \times 22 \times 4 \times 28 = \frac{2464}{6} = \frac{1232}{3} \text{ cm}^2$

Since the central angle is $60^\circ$ and the two sides are equal (radii), the triangle is equilateral. The area of an equilateral triangle is $\frac{\sqrt{3}}{4} \times \text{side}^2$.

Area of triangle = $\frac{\sqrt{3}}{4} \times 28 \times 28 = \sqrt{3} \times 7 \times 28 = 196\sqrt{3}$

Given $\sqrt{3} = 1.7$, Area of triangle = $196 \times 1.7 = 333.2 \text{ cm}^2$

Area of one design = $\frac{1232}{3} - 333.2 = 410.67 - 333.2 = 77.47 \text{ cm}^2$

Step 3: Calculate the total area of the six designs.

Total Area = $6 \times (\text{Area of one design})$

Total Area = $6 \times (\frac{1232}{3} - 333.2) = 2 \times 1232 - 6 \times 333.2$

Total Area = $2464 - 1999.2 = 464.8 \text{ cm}^2$

Step 4: Calculate the total cost.

Cost = Total Area $\times$ Rate

Cost = $464.8 \times 0.35$

Cost = $162.68$

Final Answer: The total cost of making the designs is ₹ 162.68.


More Questions from Class 10 Mathematics Areas Related to Circles EXERCISE 11.1


CBSE Solutions for Class 10 Mathematics Areas Related to Circles


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