设a∈[-2,0],已知函数 f(x)= x 3 -(a+5)x, x≤0 x 3 - a+3 2 x 2 +ax, x>0 (Ⅰ) 证明f(x)在区间(-1,1)内单调递减,在区间(1,+∞)内单调递增; (Ⅱ) 设曲线y=f(x)在点P i (x i ,f(x i ))(i=1,2,3)处的切线相互平行,且x 1 x 2 x 3 ≠0,证明 x 1 + x 2 + x 3 > 1 3 .
【简答题】Our quarrel with efficiency is not that it gets things done, but that it is a thief of time when it leaves us no leisure to enjoy ourselves, and that it strains our nerves when we try to get things do...