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  • 將函數(shù)f(x)=sin ¼x•sin ¼ (x+2π)•sin ½ (x+3π)-½cos²x/2

    將函數(shù)f(x)=sin ¼x•sin ¼ (x+2π)•sin ½ (x+3π)-½cos²x/2
    在區(qū)間(0,+∞)內(nèi)的全部極值點(diǎn)按從小到大的順序排成數(shù)列{an}(n∈N*)(1)求數(shù)列{an}的通項(xiàng)公式;(2)設(shè)bn=2nan,數(shù)列{bn}的前n項(xiàng)和為T(mén)n,求Tn的表達(dá)式.
    數(shù)學(xué)人氣:167 ℃時(shí)間:2020-05-20 19:20:58
    優(yōu)質(zhì)解答
    (1)
    f(x)=sin(x/4)•sin[(x+2π)/4]•sin [(x+3π)/2]-(1/2)cos²(x/2)
    =sin(x/4)• [cos(x/4)] • [-cos(x/2)] -(1/2)cos²(x/2)
    = -(1/2)sin(x/2)cos(x/2)-(1/2)cos²(x/2)
    =-(1/4)sinx -(1/4)(cosx+1)
    f'(x) = -(1/4)cosx + (1/4)sinx =0
    tanx =1
    x = kπ + π/4
    an = nπ + π/4
    (2)
    bn = 2nan
    = 2n(nπ + π/4)
    Tn =b1+b2+...+bn
    =2π[ (1/6)n(n+1)(2n+1) + n(n+1)/8 ]
    =(π/12)n(n+1)[ 4(2n+1)+3 ]
    =(π/12)n(n+1)(8n+7)
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