奇函数f(x)的定义域为R,且在[0,+∞)上是增函数,当0≤θ≤π/2时,是否存在实数m,使f(4m-2mcosθ)-f(2sin²θ+2)>f(0)对所有θ∈[0,π/2]求出所有适合条件的实数m.

问题描述:

奇函数f(x)的定义域为R,且在[0,+∞)上是增函数,当0≤θ≤π/2时,是否存在实数m,使f(4m-2mcosθ)-f(2sin²θ+2)>f(0)对所有θ∈[0,π/2]求出所有适合条件的实数m.

奇函数f(x) 在[0,+∞)上是增函数,则它在(-∞,0]上也是增函数,
故函数在R上是增函数。
f(-0)=-f(0), f(0)=-f(0), 2 f(0)=0, f(0)=0.
f(4m-2mcosθ)-f(2sin²θ+2)>f(0)可化为:
f(4m-2mcosθ)-f(2sin²θ+2)>0,
f(4m-2mcosθ)> f(2sin²θ+2),
4m-2mcosθ> 2sin²θ+2
m(2-cosθ)> sin²θ+1,
设2-cosθ=t∈[1,2]. cosθ=2-t, sin²θ=1-(2-t)²,
上式可化为:mt>1-(2-t)²+1,
m>(-2+4t-t²)/t=-(2/t+t)+4.
2/t+t≥2√2,-(2/t+t)+4≤-2√2+4,
m只需大于函数-(2/t+t)+4的最大值,m>-2√2+4.


由题意,f(x)在x=0处有定义且在[0,+∞)上是增函数,
故f(x)在(-∞,+∞)上连续且为增函数
由f(0)=-f(-0),得f(0)=0
f(cos2θ-3)+f(4m-2mcosθ)>f(0)=0
移向变形得
f(cos2θ-3)>-f(4m-2mcosθ)=f(2mcosθ-4m)
∴由f(x)(-∞,+∞)上连续且为增函数,得
cos2θ-3>2mcosθ-4m
2cos²θ-4-2mcosθ+4m>0
cos²θ-mcosθ+(2m-2)>0
根据题意,θ∈[0,π/2]时,cosθ∈[0,1]
令t=cosθ∈[0,1]
则,题目变成t∈[0,1]时,t²-mt+(2m-2)>0恒成立,求m的取值范围
令f(t)=t²-mt+(2m-2),此函数对应的抛物线开口向上,对称轴t=m/2,
分类讨论:
①当此抛物线对称轴t=m/2在区间[0,1]内时,m∈[0,2],
函数最小值(2m-2)-m²/4>0即可,此时m²-8m+8∴4-2√2②当对称轴在(-∞,0)时,m只要f(0)>0即可,此时2m-2>0,推出m>1,与m③当对称轴在(1,+∞)时,m>2,
只要f(1)>0即可,此时1-m+2m-2=m-1>0,推出m>1,
∴m>2
综上所述,m的取值范围是(4-2√2,+∞)

f(0) = -f(0)=> f(0) = 0f(4m-2mcosθ)-f(2sin²θ+2)>f(0)= 0=> f(4m-2mcosθ)> f(2sin²θ+2)0≤θ≤π/24m-2mcosθ > 0 and 2sin²θ+2 > 0=> 4m-2mcosθ > 2sin²θ+24m-2mcosθ > -...