Unless stated otherwise, use the approximation 22/7 for π. Class 9

Unless stated otherwise, use the approximation 22/7 for π. Class 9

Question 1.

Unless stated otherwise, use the approximation $\frac{22}{7}$ for π. Class 9

The perimeter of a circle is 44 cm. What is its radius?

Solution:

Given: Circumference of the circle (C) = 44 cm

∴ C = 2πr

∴ 2πr = 44

∴ 2 × $\frac{22}{7}$ × r = 44

∴ $\frac{44}{7}$ × r = 44

∴ r = $\frac{44 \times 7}{44} \mathrm{~cm}$

∴ r = 7cm

∴ The radius of the circle is 7 cm.

Question 2.

Calculate, correct to 3 significant figures, the circumference of a circle with: Class 9

i. radius 7 cm

ii. radius 10 cm

iii. radius 12 cm.

Solution:

i. Radius = 7 cm

∵ Circumference of circle = 2πr

= 2 × $\frac{22}{7}$ × 7 = 44 cm

∴ Circumference of circle = 44 cm

ii. Radius = 10 cm

∴ Circumference of circle = 2πr

= 2 × $\frac{22}{7}$ × 10

= $\frac{440}{7}$

= 62.857 cm

Circumference of circle = 62.9 cm (correct to 3 significant figures)

iii. Radius = 12 cm

∴ Circumference of circle = 2πr

= 2 × $\frac{22}{7}$ × 12

= $\frac{528}{7}$

= 75.428 cm

∴ Circumference of circle = 75.4 cm (correct to 3 significant figures)