Help with this question

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