题目

若tan α＜0，则（　　）                                         A．sin α＜0        B．cos α＜0                             C．sin αcosα＜0   D．sin α﹣cos α＜0                     　                                                           答案：C【考点】三角函数值的符号．                                     【专题】探究型；转化思想；分析法；三角函数的求值．                  【分析】直接由tanα＜0，可以判断sinα与cosα必定异号，从而可得答案．          【解答】解：若tanα＜0，则sinα与cosα必定异号，                  ∴sinαcosα必定小于0．                                        故选：C．                                                    【点评】本题考查了三角函数值的符号的判断，是基础题．                　                                                          

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