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$\mathrm{}=\frac{\pi \times R^{}\times n}{180}$

Millimeter (mm) Centimeter (cm) Decimeter (dm) Meter (m) Decameter (dam) Hectometer (hm) Kilometer (km)

= $\mathrm{}=\frac{\pi \times R^{}\times n}{180}=\frac{\pi \times {\mathrm{18}}^{}\times \mathrm{90}}{180}=\mathrm{9\pi \; (cm²)}$