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Q9(ii):

A brooch is made with silver wire in the form of a circle with diameter 35 mm. The wire is also used in making 5 diameters which divide the circle into 10 equal sectors as shown in Fig. 11.9. Find : (ii) the area of each sector of the brooch. (Unless stated otherwise, use $\pi = \frac{22}{7}$)

Solution :

Given:

1. The brooch is circular in shape.
2. The diameter of the circle ($d$) = $35\text{ mm}$.
3. The wire is used to form the circumference and 5 diameters.
4. The 5 diameters divide the circle into 10 equal sectors.

To Find:

The area of each sector of the brooch.

O r

Step 1: Determine the radius of the circle.

The radius ($r$) is half of the diameter ($d$).
$r = \frac{d}{2}$
$r = \frac{35}{2}\text{ mm} = 17.5\text{ mm}$

Step 2: Determine the area of the entire circle.

The formula for the area of a circle is $A = \pi r^2$.
Using $\pi = \frac{22}{7}$:
$A = \frac{22}{7} \times \left(\frac{35}{2}\right) \times \left(\frac{35}{2}\right)$
$A = \frac{22}{7} \times \frac{1225}{4}$
$A = \frac{22 \times 175}{4}$ [Since $1225 \div 7 = 175$]
$A = \frac{3850}{4} = 962.5\text{ mm}^2$

Step 3: Calculate the area of each sector.

Since the 5 diameters divide the circle into 10 equal sectors, the area of each sector is $\frac{1}{10}$ of the total area of the circle.
Area of each sector = $\frac{\text{Total Area}}{10}$
Area of each sector = $\frac{962.5}{10}$
Area of each sector = $96.25\text{ mm}^2$

Alternative Method (Using Central Angle):

The total angle of a circle is $360^\circ$.
Since there are 10 equal sectors, the central angle ($\theta$) of each sector is:
$\theta = \frac{360^\circ}{10} = 36^\circ$
Area of sector = $\frac{\theta}{360^\circ} \times \pi r^2$
Area of sector = $\frac{36}{360} \times \frac{22}{7} \times \frac{35}{2} \times \frac{35}{2}$
Area of sector = $\frac{1}{10} \times \frac{22}{7} \times \frac{1225}{4}$
Area of sector = $\frac{1}{10} \times 962.5 = 96.25\text{ mm}^2$

Final Answer: The area of each sector of the brooch is 96.25 mm².


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


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