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Mrs Dlamini buys a new lampshade for a lamp. She measures the radius of the inside circle in the lampshade to be 50 mm. The diameter of the outside (larger) circle is 40 cm. a) Calculate the circumference of the smaller, inner circle (in cm).
b) Calculate the circumference of the larger, outer circle (in cm). Round your answer to one decimal place.
c) Calculate the perimeter of half of the larger, outer circle (in cm).
d) Calculate the width of the yellow area, as shown by the horizontal line in the diagram.

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a) Radius = 50 mm = 5 cm

Circumference = π (2 x radius)
= π (2 x 5 cm)
= 3,142 x 10 cm
= 31,42 cm

b) Diameter = 40 cm
Circumference = π x diameter
= π x 40 cm
= 3,142 x 40 cm
= 125,68 cm
≈ 125,7 cm

c) We know the total circumference of the large, outer circle is 125,7 cm. The perimeter of half of the larger circle is simply the total circumference divided by 2:

125,7 cm / 2
= 62,85 cm

d) Width of yellow area = radius of large circle -  radius of small circle
= (40/2) cm -  5 cm
= 20 cm -  5 cm
= 15 cm
by Silver Status (28,731 points)

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