tolong bantu besok dikumpulkan no 12 dan 13

12)
[tex]a) \: \: 31.4 = \frac{72}{360} \times 2\pi \times r \\ r = \frac{31.4 \times 360}{72 \times 2\pi} = \frac{11.304}{452.16} = 25 \\ luas \: juring = \frac{72}{360} \times 2\pi \times 25 = 31.4 \: {cm}^{2} [/tex]
[tex]13) \: \: 231 = \frac{60}{360} \times \pi \times {r}^{2} \\ {r}^{2} = \frac{231 \times 360}{60\pi} \\ r = \sqrt{ \frac{231 \times 360 \times 7}{60 \times 22} } = \sqrt{ \frac{231 \times 42}{22} } = 21 \: cm \\ keliling = 2 \times \frac{22}{7} \times 21 = 6 \times 22 = 132 \: cm[/tex]