题目内容
已知函数().
(1)当a = 0时, 求函数的单调递增区间;
(2)若函数在区间[0, 2]上的最大值为2, 求a的取值范围.
(1)当a = 0时, 求函数的单调递增区间;
(2)若函数在区间[0, 2]上的最大值为2, 求a的取值范围.
, .
(1): 当a = 0时, f (x)=x3-4x2+5x ,
>0,
所以f (x)的单调递增区间为, .
(2)解: 一方面由题意, 得
即;
另一方面当时,
f (x) = (-2x3+9x2-12x+4)a+x3-4x2+5x ,
令g(a) = (-2x3+9x2-12x+4)a+x3-4x2+5x, 则
g(a)≤ max{ g(0), g() }
= max{x3-4x2+5x , (-2x3+9x2-12x+4)+x3-4x2+5x }
= max{x3-4x2+5x , x2-x+2 },
f (x) = g(a)
≤ max{x3-4x2+5x , x2-x+2 },
又{x3-4x2+5x}="2," {x2-x+2}="2," 且f (2)=2,
所以当时, f (x)在区间[0,2]上的最大值是2.
综上, 所求a的取值范围是.
>0,
所以f (x)的单调递增区间为, .
(2)解: 一方面由题意, 得
即;
另一方面当时,
f (x) = (-2x3+9x2-12x+4)a+x3-4x2+5x ,
令g(a) = (-2x3+9x2-12x+4)a+x3-4x2+5x, 则
g(a)≤ max{ g(0), g() }
= max{x3-4x2+5x , (-2x3+9x2-12x+4)+x3-4x2+5x }
= max{x3-4x2+5x , x2-x+2 },
f (x) = g(a)
≤ max{x3-4x2+5x , x2-x+2 },
又{x3-4x2+5x}="2," {x2-x+2}="2," 且f (2)=2,
所以当时, f (x)在区间[0,2]上的最大值是2.
综上, 所求a的取值范围是.
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