Question

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

Answer 1

　．                                                                                           

【考點】銳角三角函數的定義．                                                              

【分析】先畫出圖形，根據勾股定理和余弦函數的定義解答．                        

【解答】解：如圖：                                                                          

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

由勾股定理可得：BC=2，                                                                   

∴cosB==．                                                                            

                                                                           

【點評】本題考查銳角三角函數的定義及運用：在直角三角形中，銳角的正弦為對邊比斜邊，余弦為鄰邊比斜邊，正切為對邊比邊．