(2014•四川二模)已知函数f(x)=cos2x-sin2x+23sinxcosx.
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(2014•四川二模)已知函数f(x)=cos2x-sin2x+2
3 |
(1)∵函数f(x)=cos2x-sin2x+2
3sinxcosx
=cos2x+
3sin2x
=2sin(2x+
π
6),
∵x∈[0,
π
2],
∴(2x+
π
6)∈[
π
6,
7π
6],
∴sin(2x+
π
6)∈[-
1
2,1],
∴2sin(2x+
π
6)∈[-1,2],
∴f(x)∈[-1,2],
∴f(x)的值域[-1,2];
(2)sin(A+B)=2sin(B+C),
∴sin(π-C)=2sin(π-A),
∴sinC=2sinA,
∵sinC=
c
2R,sinA=
a
2R,(其中R为△ABC外接圆的半径),
∴c=2a,
∵
b
a=
3,
∴b=
3a,
由余弦定理,得
cosA=
b2+c2−a2
2bc=
3a2+4a2−a
3sinxcosx
=cos2x+
3sin2x
=2sin(2x+
π
6),
∵x∈[0,
π
2],
∴(2x+
π
6)∈[
π
6,
7π
6],
∴sin(2x+
π
6)∈[-
1
2,1],
∴2sin(2x+
π
6)∈[-1,2],
∴f(x)∈[-1,2],
∴f(x)的值域[-1,2];
(2)sin(A+B)=2sin(B+C),
∴sin(π-C)=2sin(π-A),
∴sinC=2sinA,
∵sinC=
c
2R,sinA=
a
2R,(其中R为△ABC外接圆的半径),
∴c=2a,
∵
b
a=
3,
∴b=
3a,
由余弦定理,得
cosA=
b2+c2−a2
2bc=
3a2+4a2−a
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