Three masses and four springs

From	Robert Jenkins <dale.jenkins8@gmail.com>
Newsgroups	comp.soft-sys.math.mathematica
Subject	Three masses and four springs
Date	2014-04-16 07:40 +0000
Message-ID	<lilc5f$qlj$1@smc.vnet.net> (permalink)
Organization	Time-Warner Telecom

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The instruction
DSolve[{-2*x1[t] + x2[t] == x1''[t], -2*x2[t] + x1[t] == x2''[t], 
  x1[0] == -1, x2[0] == 2, x1'[0] == 0, x2'[0] == 0}, {x1, x2}, t]
produces a simple solution. But I am surprised to find the three-mass version produces a mass of complication. Have I made a mistake?
DSolve[{-2*x1[t] + x2[t] == x1''[t], -2*x2[t] + x3[t] + x1[t] == 
   x2''[t], -2*x3[t] + x2[t] == x3''[t], x1[0] == -1, x2[0] == 2, 
  x3[0] == -1, x1'[0] == 0, x2'[0] == 0, x3'[0] == 0}, {x1, x2, x3},
  t]

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Three masses and four springs Robert Jenkins <dale.jenkins8@gmail.com> - 2014-04-16 07:40 +0000
  Re: Three masses and four springs Roland Franzius <roland.franzius@uos.de> - 2014-04-17 09:11 +0000

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