Regular 5-gon

Calculate the area of the regular pentagon with side 7 cm.

Correct answer:

S =  84.3 cm²

Step-by-step explanation:

$$\begin{gathered} v={\displaystyle \frac{7}{2}}cotg({\displaystyle \frac{360}{10}}) \\ {S}_1={\displaystyle \frac{1}{2}}7\cdot v \\ S=5S_1={\displaystyle \frac{5}{4}}{7}^{2}cotg(36^{\circ })=84.3{\text{cm}}^2 \end{gathered}$$

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