Re: sum of recursive fn: solving for n

*To*: mathgroup at smc.vnet.net*Subject*: [mg22111] Re: [mg22108] sum of recursive fn: solving for n*From*: Andrzej Kozlowski <andrzej at platon.c.u-tokyo.ac.jp>*Date*: Wed, 16 Feb 2000 02:34:27 -0500 (EST)*Sender*: owner-wri-mathgroup at wolfram.com

on 00.2.14 4:03 PM, fiona at reply at newsgroup.please wrote: > > what am i doing wrong here? > > f[x_] := (f[x-1])*2 > f[1] =2 > Solve[Sum[f[x], {x, 1,n}] ==62, n] > > tia, > fiona > You have to solve the recurrence equation first. You can either do it by hand, (since the answer is obvious), or use Mathematica: In[6]:= << DiscreteMath`RSolve` In[7]:= RSolve[{a[n] == 2*a[n - 1], a[1] == 2}, a[n], n] Out[7]= n {{a[n] -> 2 }} Once you know this you can use Solve: In[8]:= Solve[Sum[2^i, {i, 1, n}] == 62, n] Solve::"ifun": "Inverse functions are being used by \!\(Solve\), so some \ solutions may not be found." Out[8]= {{n -> 5}} -- Andrzej Kozlowski Toyama International University Toyama, Japan http://sigma.tuins.ac.jp/