(2009•黄冈模拟)已知函数f(x)=sin2x+2sinxsin(π2−x) +3sin2(3π2−x)
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(2009•黄冈模拟)已知函数f(x)=sin
(Ⅰ)函数f(x)=sin2x+2sinxsin(
π
2−x) +3sin2(
3π
2−x)
=sin2x+2sinxcosx+3cos2x=1+sin2x+2cos2x
=2+sin2x+cos2x=2+
2sin(2x+
π
4)
∵tan2x=
4
3,所以sin2x=
4
5,cos2x=
3
5或sin2x=−
4
5,cos2x=−
3
5
∴f(x)=
17
5或
3
5
(Ⅱ)∵x∈[0,
π
2],∴2x+
π
4∈[
π
4,
3π
4],2+
2sin(2x+
π
4)∈[1,2+
2]
所以f(x)的最大值为2+
2,最小值为1.
π
2−x) +3sin2(
3π
2−x)
=sin2x+2sinxcosx+3cos2x=1+sin2x+2cos2x
=2+sin2x+cos2x=2+
2sin(2x+
π
4)
∵tan2x=
4
3,所以sin2x=
4
5,cos2x=
3
5或sin2x=−
4
5,cos2x=−
3
5
∴f(x)=
17
5或
3
5
(Ⅱ)∵x∈[0,
π
2],∴2x+
π
4∈[
π
4,
3π
4],2+
2sin(2x+
π
4)∈[1,2+
2]
所以f(x)的最大值为2+
2,最小值为1.
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