2014-2015学年北京市顺义区八年级下期中数学试卷含答案解析  
3．一直角三角形的三边分别为2、3、x，那么以x为边长的正方形的面积为（　　）                 
A．13           B．5           C．13或5        D．无法确定
【考点】勾股定理；三角形三边关系．                                                     
【专题】分类讨论．                                                                           
【分析】以x为边长的正方形的面积即为x2．此题应考虑两种情况：2和3都是直角边或3是斜边，熟练运用勾股定理进行计算．                                                                                                
【解答】解：当2和3都是直角边时，则x2=4+9=13；                                    
当3是斜边时，则x2=9﹣4=5．                                                                
故选C．                                                                                             
【点评】本题考查的是勾股定理，此类题在没有明确直角边或斜边的时候，一定要注意分情况考虑，熟练运用勾股定理进行计算．                                                                                             
　                                                                                                       
4．菱形具有而矩形不具有的性质是（　　）                                            
A．对角线互相平分     B．四条边都相等                                            
C．对角相等                D．邻角互补                                                   
【考点】矩形的性质；菱形的性质．                                                        
【专题】证明题．                                                                               
【分析】与平行四边形相比，菱形的四条边相等、对角线互相垂直；矩形四个角是直角，对角线相等．                  
【解答】解：A、对角线互相平分是平行四边形的基本性质，两者都具有，故A不选；               
B、菱形四条边相等而矩形四条边不一定相等，只有矩形为正方形时才相等，故B符合题意；                   
C、平行四边形对角都相等，故C不选；                                                  
D、平行四边形邻角互补，故D不选．                                                     
故选：B．                                                                                          
【点评】考查菱形和矩形的基本性质．