https://tex.z-dn.net/?f=n_%7B2%7D+%3D+%5Cfrac%7B-1-+%5Csqrt%7B1%2B4.1640%7D+%7D%7B2%7D+%3D+%5Cfrac%7B-1-%5Csqrt%7B6561%7D+%7D%7B2%7D%3D+%5Cfrac%7B-1-+81+%7D%7B2%7D%3D%5Cfrac%7B-82+%7D%7B2%7D%3D-41