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问题: 集合超难题

已知f(x)是定义在(+∞,0)∪(0,+∞)上的奇函数,且在(0,+∞)上为增函数,f(1)=0.又g(θ)=sin²+mcosθ-2m(θ∈[0,π/2]﹚,若集合M=﹛m|g(θ)<0﹜。N={m▏f[g(θ)]<0﹜,求M∩N

解答:

已知f(x)是定义在(+∞,0)∪(0,+∞)上的奇函数,且在R+上为增函数,f(1)=0.又g(θ)=sin²θ+mcosθ-2m(θ∈[0,π/2]),若集合M={m|g(θ)<0}。N={m|f[g(θ)]<0},求M∩N

∵x>0时为增函数,∴由f(x)<0=f(1)--->0<x<1
∵奇函数--->x<0为增函数,∴由f(x)<0=f(-1)--->x<-1
即:f(x)<0的解集为x<-1或0<x<1
--->N={m|f[g(θ)]<0}={m|g(θ)<-1或0<g(θ)<1}
--->M∩N = {m|g(θ)<-1}
     = {m|sin²θ+mcosθ-2m<-1,0≤θ≤π/2}
     = {m|cos²θ-mcosθ+(2m-2)>0,0≤cosθ≤1}
     = {m|(t-m/2)²+(-m²/4+2m-2)>0,0≤t≤1}
     = {m≥2|t=1,t²-mt+(2m-2)>0}∪{m≤0|t=0,t²-mt+(2m-2)>0}
        ∪{0≤m≤2|t=m/2,(-m²/4+2m-2)>0}
     = {m≥2|m-1>0}∪{m≤0|2m-2>0}∪{0≤m≤2|4-2√2<m<4+2√2}
     = [2,+∞)∪空集∪(4-2√2,2]
     = (4-2√2,+∞)