maths

(1) join OB,OC,OD.
因為加左之後佢地都係等於radius.
所以會平分4隻一樣的angle

(2) ∴ ∠EOD=∠DOC=∠COB=∠BOA=45°
CO=BO
∠OCB=∠OBC=67.5° (base ∠.isos△)
BO=AO
∠OBA=∠OAB=67.5° (base ∠.isos△)
∠ABC=∠OBC+∠OBA
∠ABC=135°

(3) CO=DO
∠OCD=∠ODC=67.5° (base ∠.isos△)
DO=EO
∠ODE=∠OED=67.5 (base ∠.isos△)
∠CDE=∠ODC+∠ODE
∠CDE=135°
∴∠ABC+∠CDE=270°

都是radius，　但未必角度相等喔！