七十二角形
cos  2 π 72 = 1 4 2 ( 6 + 2 ) + i ⋅ 2 ( 6 − 2 ) 3 + 1 4 2 ( 6 + 2 ) − i ⋅ 2 ( 6 − 2 ) 3 {\displaystyle \cos {\frac {2\pi }{72}}={\frac {1}{4}}{\sqrt[{3}]{2({\sqrt {6}}+{\sqrt {2}})+i\cdot 2({\sqrt {6}}-{\sqrt {2}})}}+{\frac {1}{4}}{\sqrt[{3}]{2({\sqrt {6}}+{\sqrt {2}})-i\cdot 2({\sqrt {6}}-{\sqrt {2}})}}}