给出下列四个命题: ①命题“?x∈R,cosx>0”的否定是“?x∈R,cosx≤0”; ②若0<a<1,则函数f(x)=x 2 -a x -3只有一个零点; ③若lga+lgb=lg(a+b),则a+b的最小值为4; ④对于任意实数x,有f(-x)=f(x),且当x>0时,f′(x)>0,则当x<0时,f′(x)<0. 其中正确命题的序号是______.(填所有正确命题的序号)
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举一反三
【简答题】P assage 1 Listen to a short passage and choose the best answer to each question you hear
【简答题】Directions: Watch the video and try to understand the general idea of this p assage . For questions 1~6, mark A (for Agree)if the statement agrees with the information given in the video; B (for Disag...