Question

The diameter of a knob on a CD player is 0.78 cm. What is the circumference of the knob? What is the area of the knob?

Answer 1

Circumference of the knob `C = 2 pi r = 2 * (22/7) * 0.39 = color(blue )(2.4514)` cm

Area of the knob `A = color(blue)(0.478) color(white)(x) cm^2`

Explanation:

Given : Diameter `d = 0.78` cm

Radius `r = d / 2 = (cancel(0.78) color(red)(0.39)) / cancel( 2) = 0.39` cm

Circumference of the knob `C = 2 pi r = 2 * (22/7) * 0.39 = color(blue )(2.4514)` cm

Area of the knob `A = pi r^2 = (22/7) * (0.39)^2`

`A = (22 * 0.39 * 0.39) / 7 = color(blue)(0.478) color(white)(x) cm^2`