Carilah panjang jari* lingkaran

a. panjang jari-jari lingkaran
[tex] \frac{ \alpha }{360} = \frac{p \: busur}{2\pi \times r} \\ \frac{72}{360} = \frac{31.4}{2 \times 3.14 \times r} \\ \frac{1}{5} = \frac{31.4}{6.28 \times r} \\ 31.4 \times 5 = 6.28r \\ 157 = 6.28r \\ r = \frac{157}{6.28} = 25 \: cm[/tex]
b. luas juring
[tex] \frac{72}{360} = \frac{luas \: juring}{\pi \times r \times r} \\ \frac{1}{5} = \frac{luas \: juring}{3.14 \times 25 \times 25} \\ \frac{1}{5} = \frac{luas \: juring}{3.14 \times 25 \times 25} \\ luas \: juring \: = \frac{3.14 \times 25 \times 25}{5} \\ luas \: juring = 3.14 \times 25 \times 5 \\ luas \: juring= 392.5 \: {cm}^{2} [/tex]