江苏省无锡市江阴市XX中学2016-2017学年九年级上期中数学试卷含答案解析  
5．有下列四个命题：                                                                         
①直径是弦；                                                                                    
②经过三个点一定可以作圆；                                                                 
③三角形的外心到三角形各边的距离相等；                                            
④平分弦的直径垂直于弦．                                                                     
其中正确的有（　　）                                                                        
A．4个    B．3个      C．2个   D．1个
【考点】命题与定理．                                                                        
【分析】根据弦的定义、三角形的内心、垂径定理分别对每一项进行分析即可．                  
【解答】解：①直径是弦，故本选项正确；                                            
②经过不在同一直线的三个点可以确定一个圆，故本选项错误；                    
③三角形的内心到三角形各边的距离相等，故本选项错误；                           
④平分弦（不是直径）的直径垂直于弦，故本选项错误．                              
其中正确的有1个；                                                                           
故选D．                                                                                             
【点评】此题考查了命题与定理，用到的知识点是弦的定义、三角形的内心、垂径定理，判断命题的真假关键是要熟悉课本中的性质定理．