(2014•大兴区一模)已知函数f(x)=cos2x+2sinxcosx-sin2x
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(2014•大兴区一模)已知函数f(x)=cos2x+2sinxcosx-sin2x
(Ⅰ)求f(
(Ⅰ)求f(
π |
4 |
![(2014•大兴区一模)已知函数f(x)=cos2x+2sinxcosx-sin2x](/uploads/image/z/19228475-11-5.jpg?t=%EF%BC%882014%E2%80%A2%E5%A4%A7%E5%85%B4%E5%8C%BA%E4%B8%80%E6%A8%A1%EF%BC%89%E5%B7%B2%E7%9F%A5%E5%87%BD%E6%95%B0f%EF%BC%88x%EF%BC%89%3Dcos2x%2B2sinxcosx-sin2x)
(Ⅰ)∵f(x)=cos2x+2sinxcosx-sin2x
∴f(
π
4)=(
2
2)2+2-
2
2×
2
2-(
2
2)2=1.
(Ⅱ)∵f(x)=cos2x+2sinxcosx-sin2x=sin2x+cos2x=
2sin(2x+
π
4),
∴当-
π
2+2kπ≤2x+
π
4≤
π
2+2kπ,k∈Z,即-
3π
8+kπ≤x≤
π
8+kπ,k∈Z,函数单调增,
∴函数f(x)的单调增区间是[-
3π
8+kπ,
π
8+kπ](k∈Z),
当2x+
∴f(
π
4)=(
2
2)2+2-
2
2×
2
2-(
2
2)2=1.
(Ⅱ)∵f(x)=cos2x+2sinxcosx-sin2x=sin2x+cos2x=
2sin(2x+
π
4),
∴当-
π
2+2kπ≤2x+
π
4≤
π
2+2kπ,k∈Z,即-
3π
8+kπ≤x≤
π
8+kπ,k∈Z,函数单调增,
∴函数f(x)的单调增区间是[-
3π
8+kπ,
π
8+kπ](k∈Z),
当2x+
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