Answer:
[tex]sum = (n - 2) \times {180}^{0} \\ {1980}^{0} = (n - 2) \times {180}^{0} \\ (n - 2) = \frac{1980}{180} \\ (n - 2) = 11 \\ n = 11 + 2 \\ n = 13 \: sides[/tex]
