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Q10:

An umbrella has 8 ribs which are equally spaced (see Fig. 11.10). Assuming umbrella to be a flat circle of radius 45 cm, find the area between the two consecutive ribs of the umbrella. (Unless stated otherwise, use $\pi = \frac{22}{7}$)

Solution :

Given:

1. The umbrella is assumed to be a flat circle.
2. The radius of the circle ($r$) = $45\text{ cm}$.
3. The number of ribs in the umbrella ($n$) = $8$.
4. The ribs are equally spaced, meaning the circle is divided into $8$ equal sectors.

To Find:

The area between two consecutive ribs of the umbrella.

r = 45 cm O Sector

Step 1: Determine the central angle of each sector.

Since the umbrella is a full circle, the total angle at the center is $360^\circ$. Because there are $8$ equally spaced ribs, the circle is divided into $8$ equal sectors.

Let $\theta$ be the central angle of the sector between two consecutive ribs.

$\theta = \frac{360^\circ}{n}$

$\theta = \frac{360^\circ}{8}$

$\theta = 45^\circ$

Step 2: State the formula for the area of a sector.

The area of a sector of a circle with radius $r$ and central angle $\theta$ is given by the formula:

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

Step 3: Substitute the given values into the formula.

Given $r = 45\text{ cm}$, $\theta = 45^\circ$, and $\pi = \frac{22}{7}$:

$\text{Area} = \frac{45^\circ}{360^\circ} \times \frac{22}{7} \times (45)^2$

Step 4: Perform the arithmetic calculations.

Simplify the fraction $\frac{45}{360}$:

$\frac{45}{360} = \frac{1}{8}$

Now, calculate $45^2$:

$45 \times 45 = 2025$

Substitute these back into the area equation:

$\text{Area} = \frac{1}{8} \times \frac{22}{7} \times 2025$

$\text{Area} = \frac{1 \times 22 \times 2025}{8 \times 7}$

$\text{Area} = \frac{44550}{56}$

Divide both numerator and denominator by $2$:

$\text{Area} = \frac{22275}{28}\text{ cm}^2$

Converting to decimal form:

$\text{Area} \approx 795.5357\text{ cm}^2$

Final Answer: The area between two consecutive ribs of the umbrella is $\frac{22275}{28}\text{ cm}^2$ or approximately $795.54\text{ cm}^2$.


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


CBSE Solutions for Class 10 Mathematics Areas Related to Circles


Chapters in CBSE - Class 10 Mathematics


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