In the diagram, d_1 = 24 and d_2 = 18. What is the perimeter of the shaded region?
The shaded region has three half-circles for its perimeter.
Diameter of the circle = 24 + 18 = 42
C = pi · diameter.
Total perimeter = ½·pi·24 + ½·pi·18 + ½·pi·42 = 12·pi + 9·pi + 21·pi = 42·pi
This is the circumference of the full circle.
