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Q5(i):
In a circle of radius 21 cm, an arc subtends an angle of 60° at the centre. Find: (i) the length of the arc (Unless stated otherwise, use $\pi = \frac{22}{7}$)

Solution :

Given:

  • Radius of the circle ($r$) = $21 \text{ cm}$
  • Angle subtended by the arc at the centre ($\theta$) = $60^\circ$
  • Value of $\pi = \frac{22}{7}$

To Find:

  • The length of the arc ($l$).

60° r = 21 cm O A B

Step 1: Stating the Formula

The length of an arc ($l$) of a circle that subtends an angle $\theta$ at the centre is given by the formula:

$l = \frac{\theta}{360^\circ} \times 2\pi r$

[Where $\theta$ is the central angle in degrees, and $r$ is the radius of the circle.]

Step 2: Substituting the Given Values

Substitute $\theta = 60^\circ$, $r = 21 \text{ cm}$, and $\pi = \frac{22}{7}$ into the formula:

$l = \frac{60^\circ}{360^\circ} \times 2 \times \frac{22}{7} \times 21$

Step 3: Simplifying the Expression

First, simplify the fraction representing the portion of the circumference:

$\frac{60}{360} = \frac{1}{6}$

Now, substitute this back into the equation:

$l = \frac{1}{6} \times 2 \times \frac{22}{7} \times 21$

Step 4: Performing Arithmetic Calculations

Calculate the product of the constants:

$l = \frac{1}{6} \times 44 \times \frac{21}{7}$

[Since $2 \times 22 = 44$]

$l = \frac{1}{6} \times 44 \times 3$

[Since $21 \div 7 = 3$]

$l = \frac{132}{6}$

[Since $44 \times 3 = 132$]

$l = 22 \text{ cm}$

Final Answer: The length of the arc is 22 cm.


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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