redruby:

Help needed.

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Shift the two semicircles in order to form a full circle. Since the radius of the circle is 4cm, the diameter of the circle is 8cm. The formula for calculating the circumference of a circle is π*diameter, so the circumference of the circle that you formed with the two semicircles is 8π. So the answer is
(8π+4+4)cm=(8π+8)cm