无法证明,因为尺规作图可以作三等分角
http://baike.baidu.com/link?url=jrcEkBEX1qPC5objs1SX4nYlfcN2qGYwqPGy6g9PWjNbitafYVI4pPPPgAXfAqPHFPrcH1gECdJErG42Ji7UQuLf0cz095Tgm0PaSKXLs5PHauDYGzJkj1ZNgfqaKZwWbP1DNDcowqoewSRcoK07KGbcUUtsOUgruXYaDycklQ2DAT1tlpm9f5_wq-exAFvHIw7tmbHpS5I3-aELjyTRvJtjkMLGXrM8I-FMuK4NXkYReOD3VEpcct1v1lIMMP65
http://wenku.baidu.com/link?url=UTE9mCOb7TNCeLPOgEtqSAClzDOiD_JIiF0epb-UfYqvw-Vfp7CtODxHkDQZCwCBj_jLV6cSyPB1oYsd6dagJHU0TIin9DUj3fr-e4-fgGi