数列{an}满足an=3an-1+3^n-1,(n≥2),a4=365,若存在一个实数λ,使得{(an+λ)/3^n}为等差数列,求λ的值
人气:348 ℃ 时间:2019-10-17 06:22:40
解答
要使其为等差数列
则bn-b(n-1)为一个常数
bn-b(n-1)
=(an+λ)/3^n-[a(n-1)+λ]/3^(n-1)
把an=3a(n-1)+3^n-1代入
得:bn-b(n-1)=1-(1+2λ)/3^n
λ是实数,不能是关于n的代数式
∴1+2λ=0
∴λ=-1/2
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