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来源:百度知道 编辑:UC知道 时间:2024/06/17 14:09:47
若函数f(x)=2sin^2x -2根号3 sinxsin(x-π/2)能使得不等式
|f(x)-m|<2在区间(0,2π/3)上恒成立,则实数m的取值范围是

我是这样做的:
化简:即f(x)=1-2sin(2x+π/6)

∵|f(x)-m|<2
∴-2<1-2sin(2x+π/6)-m<2
m-3<-2sin(2x+π/6)<m+1

(0,2π/3)
所以-2sin(2x+π/6)∈【-2,2】范围
所以m-3<-2
m+1>2
错在哪里?

f(x)=2sin^2x -2根号3 sinxsin(x-π/2)
注意sin(x-π/2)=-sin(π/2-x)=-cosx
所以f(x)=2sin^2x+2根号3sinxcosx
=根号3sin2x-cosx+1
=2sin(2x-π/6)+1
所以-2<2sin(2x-π/6)+1-m<2
m-3<2sin(2x-π/6)<m+1

0<x<2π/3
-π/6<2x-π/6<7π/6
所以-1/2<sin(2x-π/6)<=1
-1<2sin(2x-π/6)<=2
所以m-3<=-1,m<=2
m+1>2,m>1
1<m<=2

f(x)=2(sinx)^2-2(根号3)sinxsin(x-π/2)=2(sinx)^2+2(根号3)sinxcosx=(根号3)sin2x-cos2x+1=2sin(2x-π/6)+1
当x∈(0,2π/3)时,-π/6<2x-π/6<7π/6,-1<2sin(2x-π/6)<2∴-m<2sin(2x-π/6)-m+1<3-m,,∴-m>-2,3-m<2,所以m∈(1,2),当m=2的时候,∴-m<2sin(2x-π/6)-m+1<3-m也成立,所以m∈(1,2】