Re: Re: Solve Equation
- To: mathgroup at smc.vnet.net
- Subject: [mg20867] Re: [mg20835] Re: [mg20813] Solve Equation
- From: Daniel Lichtblau <danl at wolfram.com>
- Date: Thu, 18 Nov 1999 01:09:42 -0500 (EST)
- References: <80qvh7$l10@smc.vnet.net> <199911170840.DAA02574@smc.vnet.net>
- Sender: owner-wri-mathgroup at wolfram.com
adam.smith at hillsdale.edu wrote: > > I believe that there was a wrong value in the set of equations given. > If I replace the L==10-6 Cos[a+b] with 13-6 Cos[a+b]: > > eqn = {L==(5 + 4Cos[a]) ,L==(13 - 12 Cos[b]), L==(13 - 6Cos[a + b])} > Out[1]= > {L==5+4 Cos[a],L==13-12 Cos[b],L==13-6 Cos[a+b]} > In[2]:= > eqn/.{L->7,b -> Pi/3, a -> 5Pi/3} > Out[2]= > {True,True,True} > > Interestingly, Solve finds a different solution that still works > > In[3]:= > mathsolve = Solve[eqn, {L, a, b}] > Solve::"ifun": > "Inverse functions are being used by \!\(Solve\), so some solutions > may \ > not be found." > Out[3]= > 1 > {{L -> 19 + 6 Sqrt[3], b -> ArcCos[- (-1 - Sqrt[3])], > 2 > > 1 > a -> ArcCos[- (7 + 3 Sqrt[3])]}} > 2 > > In[4]:= > solution = N[mathsolve] > Out[4]= > {{L -> 29.3923, b -> 3.14159 - 0.831443 I, a -> 2.49433 I}} > In[5]:= > eqn /. solution > Out[5]= > {{True,True,True}} > ... It turns out one can almost recover the desired solution (as well as several others) with just a bit of work. In[1]:= eqn = {L == 5 + 4Cos[a], L == 13 - 12 Cos[b], L == 13 - 6Cos[a + b]}; In[2]:= Timing[soln = FullSimplify[Solve[TrigToExp[eqn], {L,a,b}]];] Solve::ifun: Inverse functions are being used by Solve, so some solutions may not be found. Out[2]= {11.12 Second, Null} In[3]:= InputForm[soln] Out[3]//InputForm= {{L -> 7, b -> Pi/3, a -> -Pi/3}, {L -> 7, b -> -Pi/3, a -> Pi/3}, {L -> 19 + 6*Sqrt[3], b -> Pi + I*Log[(1 - Sqrt[2]*3^(1/4) + Sqrt[3])/2], a -> -I*Log[(7 + 3*Sqrt[3] - Sqrt[6*(12 + 7*Sqrt[3])])/2]}, {L -> 19 + 6*Sqrt[3], b -> Pi + I*Log[(1 + Sqrt[2]*3^(1/4) + Sqrt[3])/2], a -> I*Log[2/(7 + 3*Sqrt[3] + Sqrt[6*(12 + 7*Sqrt[3])])]}, {L -> 19 - 6*Sqrt[3], b -> -ArcCot[Sqrt[-1 + 2/Sqrt[3]]], a -> ArcCot[11/Sqrt[-45 + 42*Sqrt[3]]]}, {L -> 19 - 6*Sqrt[3], b -> ArcCot[Sqrt[-1 + 2/Sqrt[3]]], a -> -ArcCot[11/Sqrt[-45 + 42*Sqrt[3]]]}} Watering the first solution with an appropriately placed 2*Pi will give the desired result. As it is, we got several real-valued solutions. In[4]:= N[soln] // Chop Out[4]= {{L -> 7., b -> 1.0472, a -> -1.0472}, > {L -> 7., b -> -1.0472, a -> 1.0472}, > {L -> 29.3923, b -> 3.14159 - 0.831443 I, a -> 2.49433 I}, > {L -> 29.3923, b -> 3.14159 + 0.831443 I, a -> -2.49433 I}, > {L -> 8.6077, b -> -1.19606, a -> 0.446593}, > {L -> 8.6077, b -> 1.19606, a -> -0.446593}} Daniel Lichtblau Wolfram Research
- References:
- Re: Solve Equation
- From: adam.smith@hillsdale.edu
- Re: Solve Equation