Why Is My Solve Function Not Working in Mathematica?

[215]

I am trying to solve a function using Solve in mathmatica.
I am solving for multiple constants.

Can i use a solve on a function i have made in Mathmatica... I mean
a function like this
L2[n_, q_] :=
Total[(Range[0, IntegerPart[n/((4.1/100)/q)]]*q + 0.0038)*\[Pi]*(4.1/
100)/q] + (((IntegerPart[n/((4.1/100)/q)] + 1)*q) +
0.0038)*\[Pi]*(n - IntegerPart[n/((4.1/100)/q)]*(4.1/100)/q)

And use the solve function like this

Solve[30 == 0.0175*10^-6*L2[n, q]/((q/2)^2*\[Pi]) &&
L2[n, q] == 29.918420573509337`, {L, n}]

Or how should i use it?

 

[willem2]

You're really asking something that's too hard here. A function with a summation in it with a variable number of terms. You could try nsolve, but the only thing Mathematica could do with a discontinuous function is trying random values.

I would substitute r = n/((4.1/100)/q and then try solve for
0<=k < 1, 1 <=k <2, 2 <= k <3, ...

 

[215]

But is it posible to solve function with solve..

I mean like
L[n_,q_,E_]:= ...n...q...E
Solve[L[n_,q_,E_]==5444,{n,q,E}]

 

[willem2]

But is it posible to solve function with solve..

I mean like
L[n_,q_,E_]:= ...n...q...E
Solve[L[n_,q_,E_]==5444,{n,q,E}]

The problem is not the user defined function, but the summation with a variable number of terms. Nsolve won't work either, the only thing that might work is findroot, but you'll have to give a good enough initial quess for n and q.

 

[215]

Hmm... ok why isn't this one working then..
http://snag.gy/j41zD.jpg

The bold letters is constant..

 

[Bill Simpson]

Hmm... ok why isn't this one working then..

Probably because you have a mixture of && and ,

In[1]:= Solve[0.5 == v/R && B == \[Mu]0*v/R*n/(lj/\[Mu]j + 2*x) && 2*B^2/(2 \[Mu]0*a*(4*x/(lj/\[Mu]j + 2 x) - 1)) == 2 && R == 0.0175*10^-6*L/((g/2)^2*Pi), {L, B, v, q, n, R}]

During evaluation of In[1]:= Solve::ratnz: Solve was unable to solve the system with inexact coefficients. The answer was obtained by solving a corresponding exact system and numericizing the result. >>

During evaluation of In[1]:= Solve::svars: Equations may not give solutions for all "solve" variables. >>

Out[1]= {{L -> 4.4879895051282756*^7*g^2*R, B -> (-1.4142135623730951*Sqrt[-1.* a*lj*\[Mu]0 + 2.*a*x*\[Mu]0*\[Mu]j])/Sqrt[lj + 2.*x*\[Mu]j], v -> 0.5*R, n -> (2.*((-1.4142135623730951*lj* Sqrt[-1.*a*\[Mu]0*(lj - 2.*x*\[Mu]j)])/Sqrt[lj + 2.*x*\[Mu]j] - (2.8284271247461903*x*\[Mu]j* Sqrt[-1.*a*\[Mu]0*(lj - 2.*x*\[Mu]j)])/Sqrt[lj + 2.*x*\[Mu]j]))/(\[Mu]0*\[Mu]j)},
{L -> 4.4879895051282756*^7*g^2*R, B -> (1.4142135623730951*Sqrt[-1.* a*lj*\[Mu]0 + 2.*a*x*\[Mu]0*\[Mu]j])/Sqrt[lj + 2.*x*\[Mu]j], v -> 0.5*R, n -> (2.*((1.4142135623730951*lj*Sqrt[-1.* a*\[Mu]0*(lj - 2.*x*\[Mu]j)])/Sqrt[lj + 2.*x*\[Mu]j] + (2.8284271247461903*x*\[Mu]j*Sqrt[-1.* a*\[Mu]0*(lj - 2.*x*\[Mu]j)])/Sqrt[lj + 2.*x*\[Mu]j]))/(\[Mu]0*\[Mu]j)}}

In[2]:= Reduce[0.5 == v/R && B == \[Mu]0*v/R*n/(lj/\[Mu]j + 2*x) && 2*B^2/(2 \[Mu]0*a*(4*x/(lj/\[Mu]j + 2 x) - 1)) == 2 && R == 0.0175*10^-6*L/((g/2)^2*Pi), {L, B, v, q, n, R}]

During evaluation of In[2]:= Reduce::ratnz: Reduce was unable to solve the system with inexact coefficients. The answer was obtained by solving a corresponding exact system and numericizing the result. >>

Out[2]= (x == 0 && (B == (0. - 1.4142135623730951*I)* Sqrt[a]*Sqrt[\[Mu]0] || B == (0. + 1.4142135623730951*I)* Sqrt[a]*Sqrt[\[Mu]0]) && g != 0 && v == (1.1140846016432675*^-8*L)/g^2 && B*\[Mu]j != 0 && n == (-4.*a*lj)/(B*\[Mu]j) && R == 2.*v && L*lj != 0) || (lj + 2.*x*\[Mu]j != 0 && (B == (-1.4142135623730951* Sqrt[-1.*a*lj*\[Mu]0 + 2.*a*x*\[Mu]0*\[Mu]j])/Sqrt[lj + 2.*x*\[Mu]j] || B == (1.4142135623730951* Sqrt[-1.*a*lj*\[Mu]0 + 2.*a*x*\[Mu]0*\[Mu]j])/Sqrt[lj + 2.*x*\[Mu]j]) && g != 0 && v == (1.1140846016432675*^-8* L)/g^2 && B^2 + 2.*a*\[Mu]0 != 0 && n == (16.*a*B*x)/(B^2 + 2.*a*\[Mu]0) && R == 2.*v && L*lj - 2.*L*x*\[Mu]j != 0)

 

[215]

&& is just And, it should give a problem?

 

[Bill Simpson]

&& is just And, it should give a problem?

From your picture
Solve[expression&&expression,expression,{variablelist}]
which is not
Solve[expression&&expression&&expression,{variablelist}]
and is not
Solve[{expression,expression,expression},{variablelist}]

Read the Mathematica documentation carefully.
Mathematica does not have a "figure out what my problem actually is and what I should have asked and do that for me instead of what I actually wrote" button.

WolframAlpha.com will accept an undocumented sort of more english description of what the problem is and try to figure out what you meant and try to come up with some answer.