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  • 已知函數(shù)f(1+cotx)sinx^2-2sin(x+π/4)sin(x-π/4)

    已知函數(shù)f(1+cotx)sinx^2-2sin(x+π/4)sin(x-π/4)
    1、若tana=2,求f(a)
    2、若x∈[π/12,π/2],求f(x)的取值范圍
    數(shù)學(xué)人氣:364 ℃時間:2019-08-26 06:40:09
    優(yōu)質(zhì)解答
    (1+cotx)sin^2x=sin^2x+sinxcosx
    2sin(x+π/4)sin(x-π/4)=根號2(sinx+cosx)*根號2/2(sinx-cosx)=sin^2x-cos^2x
    f(x)=(1+cotx)sinx^2-2sin(x+π/4)sin(x-π/4)=sinxcosx+cos^2x
    f(x)=1/2sin2x+1/2(2cos^2x-1)+1/2
    f(x)=1/2[sin2x+cos2x]+1/2
    f(x)=根號2/2sin(2x+π/4)+1/2
    1.tana=2=> sina/cosa=2 sin^2a+cos^2a=1=>cos^2a=1/5
    f(a)=sinacosa+cos^2a=3cos^2a=3/5
    2.x∈[π/12,π/2],2x屬于[π/6,π]
    2x+π/4屬于[5/12π,5/4π]
    sin(2x+π/4)屬于[根號2/2,1]
    f(x)取值范圍為[1,(根號2+1)/2]
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