Unsur Lingkaran
- Jari-jari ($r$), diameter ($d = 2r$).
- Busur, juring, tembereng.
Keliling dan Luas
$$K = 2\pi r \quad \text{atau} \quad K = \pi d$$ $$L = \pi r^2$$
Contoh: $r = 7$, $\pi = \frac{22}{7}$: $$K = 2 \times \frac{22}{7} \times 7 = 44, \quad L = \frac{22}{7} \times 7^2 = 154$$
Panjang Busur
$$\text{Busur} = \frac{\alpha}{360°} \times 2\pi r$$
Luas Juring
$$\text{Juring} = \frac{\alpha}{360°} \times \pi r^2$$
dengan $\alpha$ besar sudut pusat.