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$=\sqrt{a^2+c^2-2ac.\cos \alpha }$

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

= $\sqrt{a^2+c^2-2\mathrm{a . }c.\cos \alpha }(cm²)$
= $\sqrt{9^2+5^2-\mathrm{2\; .}\mathrm{9 . }5.\cos \mathrm{90}}(cm²)$
= $\sqrt{\mathrm{146,32}}(cm²)$
= $\mathrm{12,10\; (cm²)}$