円周率の近似
π 4 = 12 arctan  1 18 + 8 arctan  1 57 − arctan  1 239 {\displaystyle {\frac {\pi }{4}}=12\arctan {\frac {1}{18}}+8\arctan {\frac {1}{57}}-\arctan {\frac {1}{239}}}