求数列{(2n+1)*(1/3^n)}的前几项和
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求数列{(2n+1)*(1/3^n)}的前几项和
求数列{(2n+1)*(1/3^n)}的前几项和
求数列{(2n+1)*(1/3^n)}的前几项和
Sn =(2*1+1)*1/3 +(2*2+1)*1/3^2 + .+ (2n+1)*1/3^n
Sn+1 =(2*1+1)*1/3 +(2*2+1)*1/3^2 + .+ (2n+1)*1/3^n+(2n+3)*1/3^(n+1)
Sn+1 -1/3 Sn =(2*1+1)*1/3 +2*( 1/3^2+1/3^3+ .+1/3^(n+1) )
=1/3 +2*( 1/3 + 1/3^2+1/3^3+ .+1/3^(n+1) ) ------------(1式)
又由于 Sn+1 -1/3 Sn= Sn+An+1 -1/3 Sn
=2/3Sn +(2n+3) 1/3(n+1) ---------------------------------------------(2式0
(1)(2)式可解 Sn
an=1/[(2n+1)(2n+3)]=[(2n+3)-(2n+1)]/[2(2n+1)(2n+3)]=(2n+3)/[2(2n+1)(2n+3)]-(2n+1)/[2(2n+1)(2n+3)]=1/[2(2n+1)]-1/[2(2n+3)]=(1/2)[1/(2n+1)-1/(2n+3)]...
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an=1/[(2n+1)(2n+3)]=[(2n+3)-(2n+1)]/[2(2n+1)(2n+3)]=(2n+3)/[2(2n+1)(2n+3)]-(2n+1)/[2(2n+1)(2n+3)]=1/[2(2n+1)]-1/[2(2n+3)]=(1/2)[1/(2n+1)-1/(2n+3)]
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