已知函数f(x)=2cosxsin(x+π/3)-√3sin²x+sinxcosx(x属于R)

问题描述:

已知函数f(x)=2cosxsin(x+π/3)-√3sin²x+sinxcosx(x属于R)
1 函数的最大值及取得最大值时的x
2 函数的单调减区间
急求各位大大了
1个回答 分类:数学 2014-10-25

问题解答:

我来补答
解∵f(x)=2cosx[sinxcos(π/3)+cosxsin(π/3)]-√3(sinx)^2+sinxcosx
=sinxcosx+√3(cosx)^2-√3(sinx)^2+sinxcosx
=sin2x+√3cos2x
=2[(1/2)sin2x+(√3/2)cos2x]
=2[sin2xcos(π/3)+cos2xsin(π/3)]
=2sin[2x+(π/3)]
∴当2x+(π/3)=π/2时,函数f(x)有最大值且f(x)=2,此时,x=π/12;
当2x+(π/3)=3π/2时,函数f(x)有最小值且f(x)=-2,此时,x=7π/12;
又∵sinx的单调减区间为:2kπ+(π/2)≤x≤2kπ+(3π/2);
∴2kπ+(π/2)≤2x+(π/3)≤2kπ+(3π/2)
解之得:kπ+(π/12)≤x≤kπ+(7π/12)
 
 
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