The formula for perimeter/circumference is \(d\pi\), where \(d\) is the diameter.

a) \(d = 12\), so the circumference is \(12\pi\). Since the question is asking for a semicircle, we divide \(12\pi\) by \(2\) to get \(6\pi\) mm. The question is asking to round your answer, so we can get \(18.85\) mm.

b) If \(r = 4\), then \(d = 8\). We do the same thing for this problem as we did the last. The circumference for the semicircle would be \(4\pi\) ft. This multiplied by \(\pi\) rounds to \(12.57\) ft.

c) The question does not specify is \(d = 9\) or \(r = 9\). If \(d = 9\), the circumference would be \(4.5\pi\) m. If \(r = 9\), the circumference would be \(9\pi\) m. This will give us \(28.27\) m.

d) This question does not specify whether \(d = 11.5\) or \(r = 11.5\), as well. If \(d = 11.5\), the circumference for the semicircle would be \(5.75\pi\) in. If \(r = 11.5\), the circumference for the semicircle would be \(11.5\pi\) in. So our answer would be \(36.13\) in.

- Daisy