Question

12. If the concentric circles have radii 350 m and 490 m, then the difference in their circumferences
(a) 960 m
(b) 990 m
(c)890 m
(d)none of these​

Answer 1

Given :

If the concentric circles have radii 350 m and 490 m.

To Find :

The difference in their circumferences.

Solution :

Let R = 490

r = 350

Difference = 2π[490 - 350]

= 2 × 22/7 × 140

= 2 × 22 × 20

= 880m

Hence, none of these.

The answer is 880m.

Answer 2

Given:

If the concentric circles have radii 350 m and 490 m, then the difference in their circumferences

Solution:

Know that,

Circumference of the circle$\[ = 2\pi r\]$

Find the difference in their circumferences,

$\[\begin{array}{l} = \left( {2\pi \times 490} \right) - \left( {2\pi \times 350} \right)\\ = 2 \times \frac{{22}}{7} \times \left( {490 - 350} \right)\\ = 2 \times \frac{{22}}{7} \times 140\\ = 880\,{\rm{m}}\end{array}\]$

Hence, the correct answer is option(d) i.e. none of these.