题目

如图5所示,四边形ABCD是⊙O的内接四边形,延长BC到E,已知∠BCD: ∠ECD=3:2,那么 ∠BOD=_______. 答案：144° 解:∵∠BCD:∠ECD=3:2, 设∠BCD=3k,则∠ECD=2k, ∵∠BCD+∠ECD=180°,∴3k+2k=180°,∴k=36°,∴∠BCD=108°,∠ECD=72°, ∴∠A= 72°,∴∠BOD=144°.       点拨:此题由圆的有关性质及圆周角性质来解决,易将圆周角性质与圆心角性质发生混淆. If law and order as well as rules and regulations ______, neither the citizen nor his property is safe. A. are not observed                         B. is not observed      C. were not observed                        D. was not observed