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MathGroup Archive 2005

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Re: How to compute this sum?

  • To: mathgroup at smc.vnet.net
  • Subject: [mg63066] Re: [mg63041] How to compute this sum?
  • From: Bob Hanlon <hanlonr at cox.net>
  • Date: Tue, 13 Dec 2005 03:40:48 -0500 (EST)
  • Reply-to: hanlonr at cox.net
  • Sender: owner-wri-mathgroup at wolfram.com

expr=(-6*2^n*n+3*2^n+2*3^n*n)/(3^n)/(n^2-3*n+2)/(2^n);

expr//FullSimplify

(2*(-3*2^n + 3^n)*n + 3*2^n)/(6^n*((n - 2)*(n - 1)))

The term is undefined for n equal to either 1 or 2. Excluding those terms,

Sum[Evaluate[expr],{n,3,Infinity}]

1/6


Bob Hanlon

> 
> From: "mizhael" <loseminds at hotmail.com>
To: mathgroup at smc.vnet.net
> Date: 2005/12/11 Sun PM 10:25:16 EST
> Subject: [mg63066] [mg63041]  How to compute this sum?
> 
> sum((-6*2^n*n+3*2^n+2*3^n*n)/(3^n)/(n^2-3*n+2)/(2^n),n = 1 .. Inf)
> 
> thanks a lot! 
> 
> 
> 


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