Re: Problem solving equation
- To: mathgroup at smc.vnet.net
- Subject: [mg40744] Re: [mg40741] Problem solving equation
- From: Andrzej Kozlowski <akoz at mimuw.edu.pl>
- Date: Wed, 16 Apr 2003 01:34:43 -0400 (EDT)
- Sender: owner-wri-mathgroup at wolfram.com
1. Log[a+b] does not equal Log[a]+ Log[b] 2. 1.046*Exp[.0151*t] + 1.284*Exp[.0087*t]/.t->64.94 5.04776 which is rather far from 6.3 3. Solve and NSolve are meant for solving equations involving polynomials and rational functions or equations that are "essentially" of this type. Andrzej Kozlowski Yokohama, Japan http://www.mimuw.edu.pl/~akoz/ http://platon.c.u-tokyo.ac.jp/andrzej/ On Tuesday, April 15, 2003, at 05:01 pm, Heath wrote: > Hi, > I'm trying to solve the following equation: > > Solve[1.046*Exp[.0151*t] + 1.284*Exp[.0087*t] == 6.3, t] > > and am getting this error message: > > Solve::"tdep": "The equations appear to involve the variables to be > solved \ > for in an essentially non-algebraic way." > > I cannot solve it with Solve[] or NSolve[]. > I was able to find the solution by using Plot[] to get an idea where > the solution is, followed by FindRoot[]. > > Why doesn't the expression simplify to, as I believe it does on paper, > to: > Log[1.046*Exp[.0151*t]] + Log[1.284*Exp[.0087*t]] == Log[6.3] > Log[1.046]+Log[Exp[.0151*t]]+Log[1.284]+Log[Exp[.0087*t]]==Log[6.3] > Log[1.046*1.284]+.0151*t+.0087*t==Log[6.3] > .0238*t==Log[6.3]-Log[1.046*1.284] > .0238*t==Log[6.3/(1.046*1.284)] > .0238*t==1.545596 > t==64.94 > > This "hand" solution is not the same as the solution I get using > FindRoot[] in Mathematica. Mathematica finds the solution is: > t==82.9182 > > > Where am I making an error? Is my hand calculation wrong, or is > Mathematica wrong? Why am I getting an error message using Solve[] or > NSolve[] with Mathematica? > > Thanks, > Heath > > >