Miara kąta wewnętrznego n-kąta foremnego wyraża się wzorem: $\frac{n-2}{n}\cdot {180}^{\circ }$

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Oznaczmy:

α - miara kąta wewnętrznego n-kąta foremnego

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$\text{a)}\alpha =\frac{n-2}{n}\cdot {180}^{\circ }=\frac{9-2}{9}\cdot {180}^{\circ }=\frac{7}{9}\cdot {180}^{\circ }=7\cdot {20}^{\circ }={140}^{\circ }$  

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$\text{b)}\alpha =\frac{n-2}{n}\cdot {180}^{\circ }=\frac{18-2}{18}\cdot {180}^{\circ }=\frac{16}{18}\cdot {180}^{\circ }=16\cdot {10}^{\circ }={160}^{\circ }$  

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$\text{c)}\alpha =\frac{n-2}{n}\cdot {180}^{\circ }=\frac{45-2}{45}\cdot {180}^{\circ }=\frac{43}{45}\cdot {180}^{\circ }=43\cdot 4^{\circ }={172}^{\circ }$  

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$\text{d)}\alpha =\frac{n-2}{n}\cdot {180}^{\circ }=\frac{900-2}{900}\cdot {180}^{\circ }=\frac{898}{900}\cdot {180}^{\circ }={\left(\frac{898}{5}\right)}^{\circ }={179,6}^{\circ }$