已知向量a=(根号3sinx,cosx),b=(cosx,cosx),函数f(x)=a·b-½

2个回答

  • 1

    a·b=√3sinxcosx+cosx^2=√3sin(2x)/2+(1+cos(2x))/2

    =sin(2x+π/6)+1/2

    故:f(x)=a·b-1/2=sin(2x+π/6)

    最小正周期:T=2π/2=π

    2

    f(α)=sin(2α+π/6)=4/5

    π/6≤α≤5π/12,即:π/2≤2α+π/6≤π

    故:cos(2α+π/6)=-3/5

    sin(2α)=sin(2α+π/6-π/6)

    =sin(2α+π/6)cos(π/6)-cos(2α+π/6)sin(π/6)

    =(4/5)(√3/2)-(-3/5)(1/2)=(3+4√3)/10

    3

    f(x)图像向右平移π/6个单位,得到:sin(2x-π/6)

    即:g(x)=sin(2x-π/6)

    x∈[0,π/2],即:2x-π/6∈[-π/6,5π/6]

    g(x)=k在x∈[0,π/2]上有一个实根

    即:sin(2x-π/6)=k在x∈[0,π/2]上有一个实根

    sin(2x-π/6)∈[-1/2,1]

    k=1时,只有一个实根

    -1/2≤k