f(x)=x^3-3ax,g(x)=lnx,(1)当a=1,求 f(x)在区间[-2,2]上的最小值(2)若在区间[1,2]上f(x) 的图象恒在g(x)图象的上方,求实数a的取值范围(3) 求f(x) 在区间[-1,1]上的最大值F(a)的解析式
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f(x)=x^3-3ax,g(x)=lnx,(1)当a=1,求 f(x)在区间[-2,2]上的最小值(2)若在区间[1,2]上f(x) 的图象恒在g(x)图象的上方,求实数a的取值范围(3) 求f(x) 在区间[-1,1]上的最大值F(a)的解析式
f(x)=x^3-3ax,g(x)=lnx,(1)当a=1,求 f(x)在区间[-2,2]上的最小值
(2)若在区间[1,2]上f(x) 的图象恒在g(x)图象的上方,求实数a的取值范围
(3) 求f(x) 在区间[-1,1]上的最大值F(a)的解析式
f(x)=x^3-3ax,g(x)=lnx,(1)当a=1,求 f(x)在区间[-2,2]上的最小值(2)若在区间[1,2]上f(x) 的图象恒在g(x)图象的上方,求实数a的取值范围(3) 求f(x) 在区间[-1,1]上的最大值F(a)的解析式
(1) 当a=1,函数f (x)=x^3-3ax在区间[-2,2]上连续,因此可导,f′(x) =3x^2-3a=3(x^2-1),f (x)的驻点为x=±1,当x=1时,f (x) =-2,当x=-1时,f (x) =2,而x=-2时,f (x) =-2,x=2时,f (x) =2,故f (x) 在区间[-2,2]上的最小值为-2
(2)若在区间[1,2]上f(x) 的图象恒在g(x)图象的上方,则x^3-3ax>lnx,即e^〔x(x^2-3a)〕>x,当x=1时,f (x) =e^(1-3a),当x=2时,f (x) e^(8-3a),e^〔x(x^2-3a)〕和x均为单调增加的函数,由e^(1-3a)>1得,a<1/3,e^(8-3a)<2得,a<(8-ln2)/3,而1/3<(8-ln2)/3,由此得在区间[1,2]上e^〔x(x^2-3a)〕比x增长速度快,因此a<1/3.
(3)在区间[-1,1]上,f′(x) =3(x^2-a),f〃(x) =6 x,由f′(x) =0得x=±√ a(0≤a≤1),由f〃(x)<0得x<0,因此x=√ a时f (x)取得最大值,最大值F(a) =a^(3/2)-3 a^(3/2)=-2 a^(3/2)
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