题目

讨论函数f(x)=x3-3x2-9x+1在区间［-4,4］上的单调性. 答案：解:f′(x)=3x2-6x-9,令f′(x)≥0,结合-4≤x≤4,得-4≤x≤-1或3≤x≤4;令f′(x)≤0,结合-4≤x≤4,得-1≤x≤3.∴f(x)在［-4,-1］,［3,4］上为增函数,在［-1,3］上为减函数.4. For those with family members far away, the personal computer and the phoneare important in staying    .A. connected       B. connectingC. to connect     D.  to be connected