wxMaxima extract solution from "solve"

Question

I have the following code which works fine untill the second to last instruction.

(%i17)  M_A;

(%o17)  [500.0,-0.22*F_Bz-750.0,0.22*F_By-105.0]

(%i18)  solve(M_A[2]);

rat: replaced -750.0 by -750/1 = -750.0

rat: replaced -0.22 by -11/50 = -0.22

(%o18)  [F_Bz=-37500/11]

(%i19)  solve(M_A[3]);

rat: replaced -105.0 by -105/1 = -105.0

rat: replaced 0.22 by 11/50 = 0.22

(%o19)  [F_By=5250/11]

(%i20)  F_Br : sqrt(F_By^2+F_Bz^2);

(%o20)  sqrt(F_Bz^2+F_By^2)

(%i21)  float(%);

(%o21)  sqrt(F_Bz^2+F_By^2)

how can I obtain a numerical answer for the %i20.

@RobertDodier it only prints F_Br, without assigning anything to it — mily, Oct 30 '21 at 07:22
Meanwhile, I have tried: `tmp1 : M_A[2];` which produces `-0.22*F_Bz-750.0` then `tmp2 : solve(tmp1);` produces `[F_Bz=-37500/11]` finally `tmp3 : rhs(%);` spits out `0` instead of -37500/11. How can I obtain -37500/11 assigned to tmp3 — mily, Oct 30 '21 at 07:38
I'm not sure I understand what you mean when you say "it only prints F_Br, without assigning anything to it" -- I tried the idea I suggested and it seems to have the intended effect. Maybe you can show the inputs you tried. — Robert Dodier, Oct 31 '21 at 00:55
About the `tmp3`, note that `%` is a list with one element, which is an equation. I think `tmp3: rhs(%[1]);`, i.e. obtain the first (and only) element, and then take the rhs of that, would have the effect you were expecting. — Robert Dodier, Oct 31 '21 at 03:37

score 1 · Accepted Answer · answered Oct 31 '21 at 00:53

Here's what I get when I try substituting the solutions of the equations into the expression for F_Br. See %i7 and %o7.

(%i3) M_A:[500.0,(-0.22)*F_Bz-750.0,0.22*F_By-105.0]
(%o3)   [500.0, (- 0.22 F_Bz) - 750.0, 0.22 F_By - 105.0]
(%i4) s1:solve(M_A[2])

rat: replaced -750.0 by -750/1 = -750.0

rat: replaced -0.22 by -11/50 = -0.22
                                  37500
(%o4)                   [F_Bz = - -----]
                                   11
(%i5) s2:solve(M_A[3])

rat: replaced -105.0 by -105/1 = -105.0

rat: replaced 0.22 by 11/50 = 0.22
                                  5250
(%o5)                     [F_By = ----]
                                   11
(%i6) F_Br:sqrt(F_By^2+F_Bz^2)
                                2       2
(%o6)                  sqrt(F_Bz  + F_By )
(%i7) subst(append(s1,s2),F_Br)
                         750 sqrt(2549)
(%o7)                    --------------
                               11
(%i8) float(%)
(%o8)                   3442.337880372093

wxMaxima extract solution from "solve"

1 Answers1