_1和formula_45的交点。那么formula_46, formula_50, formula_54, formula_58四点共圆当且仅当formula_47, formula_51, formula_55, formula_59四点共圆。五圆定理：设formula_70为任意五边形，五点formula_71, formula_72, formula_73, formula_74, formula_75分别是formula_76和formula_30 , formula_29和formula_7