∶pco 절단 ⊙ O ~ c, ∰pco = 90, ∰oce = ∰pco-∰pcd = 90-∰pcd.
∶pc = PD, ∰pcd = ∰pdc, ∰oce = 90-∰pdc.
분명히: "PDC =" ode, "oce = 90-"ode, 그리고 OC = OD, "oce =" OEC,
∰oec = 90-ode, OEC =180-doe-ode, doe = 90
∮ AE = be, ∮ Abe = ∮ BCE.