(2014•江门一模)已知函数f(x)=4cosxsin(x+[π/6])-1,x∈R.

1个回答

  • 解题思路:(1)依题意,可知f(0)=1;

    (2)利用北京公式与辅助角公式可求得f(x)=2sin(2x+[π/6]),f(x-ϕ)=2sin(2x+[π/6]-2ϕ),利用y=2sin(2x+[π/6]-2ϕ)经过坐标原点,ϕ>0,即可求得ϕ的最小值.

    (1)∵f(x)=4cosxsin(x+[π/6])-1,

    ∴f(0)=4cos0sin[π/6]-1=1;

    (2)∵f(x)=4cosxsin(x+[π/6])-1

    =4cosx(

    3

    2sinx+[1/2]cosx)-1

    =2

    3sinxcosx+2cos2x-1

    =

    3sin2x+cos2x

    =2sin(2x+[π/6]),

    ∴f(x-ϕ)=2sin[2(x-ϕ)+[π/6]]=2sin(2x+[π/6]-2ϕ),

    ∵y=2sin(2x+[π/6]-2ϕ)经过坐标原点,

    ∴[π/6]-2Φ=kπ(k∈Z),

    ∴ϕ=[π/12]-[kπ/2](k∈Z),又ϕ>0,

    ∴当k=0时,ϕ取得最小值,为[π/12].

    点评:

    本题考点: 三角函数中的恒等变换应用;函数y=Asin(ωx+φ)的图象变换.

    考点点评: 本题考查三角函数中的恒等变换应用,求得f(x)=2sin(2x+[π/6])是关键,考查函数y=Asin(ωx+φ)的图象变换,考查正弦函数的性质,属于中档题.