(2010•内江二模)已知函数f(x)=2cosxsin(x+π3)−3sin2x+sinxcosx
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(2010•内江二模)已知函数f(x)=2cosxsin(x+
)−
sin
π |
3 |
3 |
(Ⅰ)f(x)=2cosx(
1
2sinx+
3
2cosx)−
3sin2x+sinxcosx
=2sinxcosx+
3(cos2x−sin2x)
=sin2x+
3cos2x
=2sin(2x+
π
3)
∴T=π
(Ⅱ)f(x)的减区间为2kπ+
π
2≤2x+
π
3≤2kπ+
3π
2,kπ+
π
12≤x≤kπ+
7π
12
又∵x∈[−
π
2,−
π
12],∴−
π
2≤x≤−
5π
12或
π
12≤x≤
π
2
即f(x)在[−
π
2,−
5π
12]和在[
π
12,
π
2]上单调递减.
1
2sinx+
3
2cosx)−
3sin2x+sinxcosx
=2sinxcosx+
3(cos2x−sin2x)
=sin2x+
3cos2x
=2sin(2x+
π
3)
∴T=π
(Ⅱ)f(x)的减区间为2kπ+
π
2≤2x+
π
3≤2kπ+
3π
2,kπ+
π
12≤x≤kπ+
7π
12
又∵x∈[−
π
2,−
π
12],∴−
π
2≤x≤−
5π
12或
π
12≤x≤
π
2
即f(x)在[−
π
2,−
5π
12]和在[
π
12,
π
2]上单调递减.
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