# Ex.11.3 Q15 Perimeter and Area - NCERT Maths Class 7

## Question

Find the circumference of the inner and the outer circles, shown in the adjoining figure? (Take \(π = 3.14\))

## Text Solution

**What is known?**

Inner and outer circles of radius \(10 \rm\,m\) and 1\(9 \rm\,m\).

**What is unknown?**

The circumference of the inner and the outer circles.

**Reasoning:**

This question is straight forward, use the given information as the inner and outer radius of the circle is given \(10 \rm\,m\) and 1\(9 \rm\,m\), by using formula of the circumference of the circle you can find out the circumference of the inner and outer circles.

**Steps:**

Radius of the outer circle \((r_1) = 19 \rm\,m\)

Therefore,

Circumference of outer circle

\[\begin{align}&= 2\pi {r_1} \\&= 2 \times 3.14 \times 19\\ &= 119.32{\rm{ }}\rm\,m\end{align}\]

Radius of inner circle\((r_2)\)

\(\begin{align} &= 19 \,– 10 \\&= 9\rm\, m\end{align}\)\(\)

Therefore,

Circumference of outer circle

\[\begin{align} &= 2\pi {r_2}\\ &= 2 \times 3.14 \times 9\\ &= 56.52{\rm{ }}\rm \,m\end{align}\]

Therefore, the circumferences of inner and outer circles are \(56.52 \rm\,m\) and \(119.32 \rm\,m\) respectively.