Ex.11.3 Q1 Perimeter and Area - NCERT Maths Class 7

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Question

Find the circumference of the circles with the following radius:

\(\begin{align}\left( {{\rm{Take}}\;\pi = \frac{{22}}{7}} \right)\end{align}\)

(a) \(14 \,\rm cm\)   

(b) \(28 \,\rm mm \)    

(c) \(21 \,\rm cm\)

Text Solution

What is known?

Radius of the circle

What is unknown?

The circumference of the circles.

Reasoning:

In this question, radius of the circle is given. Put the value of the radius in the formula of circumference of the circle to find out the circumference.

Steps:

(a) Circumference of the circle

\[\begin{align} &= 2\pi r\\&= 2 \times \frac{{22}}{7} \times 14\\&= 2 \times 22 \times 2\\&= 88\;\rm{cm}\end{align}\]

(b) Circumference of the circle

\[\begin{align}&= 2\pi r\\&= 2 \times \frac{{22}}{7} \times 28\\&= 2 \times 22 \times 4\\&= 176\;\rm{mm}\end{align}\]

(c) Circumference of the circle

\[\begin{align}&= 2\pi r\\&= 2 \times \frac{{22}}{7} \times 21\\&= 2 \times 22 \times 3\\&= 132\;\rm{cm}\end{align}\]