题目

函数f(x)=ax3+x+1有极值的充要条件是(　　)A.a＞0                                              B.a≥0C.a＜0                                              D.a≤0 答案：解析：令f′(x)=3ax2+1=0,得x2=∴＞0，即a＜0.故选C.答案：C

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