四十九角形
cos  2 π 49 = cos  2 π 7 ⋅ 7 = 1 2 ( cos  2 π 7 + i ⋅ sin  2 π 7 7 + cos  2 π 7 − i ⋅ sin  2 π 7 7 ) = 1 2 ( cos  2 π 7 + i ⋅ 1 − cos 2  2 π 7 7 + cos  2 π 7 − i ⋅ 1 − cos 2  2 π 7 7 ) {\displaystyle {\begin{aligned}\cos {\frac {2\pi }{49}}=&\cos {\frac {2\pi }{7\cdot 7}}\\=&{\frac {1}{2}}\left({\sqrt[{7}]{\cos {\frac {2\pi }{7}}+i\cdot \sin {\frac {2\pi }{7}}}}+{\sqrt[{7}]{\cos {\frac {2\pi }{7}}-i\cdot \sin {\frac {2\pi }{7}}}}\right)\\=&{\frac {1}{2}}\left({\sqrt[{7}]{\cos {\frac {2\pi }{7}}+i\cdot {\sqrt {1-\cos ^{2}{\frac {2\pi }{7}}}}}}+{\sqrt[{7}]{\cos {\frac {2\pi }{7}}-i\cdot {\sqrt {1-\cos ^{2}{\frac {2\pi }{7}}}}}}\right)\\\end{aligned}}}
cos  ( 2 π / 7 ) {\displaystyle \cos(2\pi /7)} は正七角形も参照