Question

在△ABC中，∠C=90°，AC=1，AB=3，则cosB=　　　　　　．                    

Answer 1

　．                                                                                           

【考点】锐角三角函数的定义．                                                              

【分析】先画出图形，根据勾股定理和余弦函数的定义解答．                        

【解答】解：如图：                                                                          

在△ABC中，∠C=90°，AC=1，AB=3，                                                  

由勾股定理可得：BC=2，                                                                   

∴cosB==．                                                                            

                                                                           

【点评】本题考查锐角三角函数的定义及运用：在直角三角形中，锐角的正弦为对边比斜边，余弦为邻边比斜边，正切为对边比边．