Re: sum of recursive fn: solving for n
- To: mathgroup at smc.vnet.net
- Subject: [mg22123] Re: sum of recursive fn: solving for n
- From: "Robert Nowak" <robert.nowak at ims.co.at>
- Date: Wed, 16 Feb 2000 02:34:39 -0500 (EST)
- Organization: telecom.at (Vienna, Austria)
- References: <888afd$c7p@smc.vnet.net>
- Sender: owner-wri-mathgroup at wolfram.com
your function f[x_] seems to be 2^x so the following will solve your problem
In[1]:=
f[x_] = 2^x;
In[2]:=
g[n] = Sum[f[x], {x, 1, n}]
Out[2]=
2*(-1 + 2^n)
In[3]:=
Solve[g[n] == 62, n]
Solve::"ifun" : "Inverse functions are being used by \!\(Solve\), so some \
solutions may not be found."
Out[3]=
{{n -> 5}}
hope that helps
Robert
--
---
Robert Nowak (robert.nowak at ims.co.at)
Ionen Mikrofabrikations Systeme GmbH
A-1020 Wien, Schreygasse 3, Austria
Phone: (+43 1)2144894-32, Fax: (+43 1)2144894-99
fiona <reply at newsgroup.please> wrote in message
news:888afd$c7p at smc.vnet.net...
>
> 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
>