# Solve function not working why?

1. May 11, 2013

### 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?

2. May 12, 2013

### 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, ....

3. May 12, 2013

### 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}]

4. May 12, 2013

### willem2

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.

5. May 12, 2013

### 215

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

The bold letters is constant..

6. May 13, 2013

### Bill Simpson

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)

7. May 13, 2013

### 215

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

8. May 13, 2013