# Solving 3 simultaneous equations using solve.

6 views (last 30 days)
Karien Bester on 2 Sep 2021
Answered: Wan Ji on 2 Sep 2021
Good day,
I am struggling to use the solve function in order to solve for L, Rs, and Rb. My code does not produce any results, or errors. Please find my code below. I would appreciate any advice.
Karien
%define attributes:
gamma=21.05;
phi=35.55;
phi1=0.75*phi;
d=0.6;
syms L Rs Rb;
zc=(0.3*phi1-3.5)*d;
Ab=0.2827; %'pi'*(d*d)/4;
As1=5.08796;%'pi'*d*zc;
As2='pi'*d*(L-zc);
Nq=exp('pi'*tan(phi))*(tan(phi/2 +45))^2;
svb=gamma*L;
svs=gamma*zc;
G=1200;
Q=600;
eq1= Rs/1.4 + Rb/1.7 == 1.0*G +1.3*Q;
eq2= Rs == svs*tan(phi)*(1-sin(phi))*(0.5*As1+As2);
eq3= Rb == svb*Ab*Nq;
sol= solve([eq1,eq2,eq3],[L,Rs,Rb]);
Lsol=sol.L;
Rbsol=sol.Rb;

Wan Ji on 2 Sep 2021
Do not use 'pi' use pi instead
%define attributes:
gamma=21.05;
phi=35.55;
phi1=0.75*phi;
d=0.6;
syms L Rs Rb;
zc=(0.3*phi1-3.5)*d;
Ab=0.2827; %'pi'*(d*d)/4;
As1=5.08796;%'pi'*d*zc;
As2=pi*d*(L-zc);
Nq=exp(pi*tan(phi))*(tan(phi/2 +45))^2;
svb=gamma*L;
svs=gamma*zc;
G=1200;
Q=600;
eq1= Rs/1.4 + Rb/1.7 == 1.0*G +1.3*Q;
eq2= Rs == svs*tan(phi)*(1-sin(phi))*(0.5*As1+As2);
eq3= Rb == svb*Ab*Nq;
sol= solve([eq1,eq2,eq3],[L,Rs,Rb])
Lsol=sol.L
Rbsol=sol.Rb
Result is
Lsol =
(255*(4275354068281784437756792116609024*pi + 39038561809674979475803485110635625))/(274877906944*(1469363753180227599360000*pi + 29915351642033550057827))
(433291241555137581*(464595761707792941092070619575922905317376*pi + 1390999186121382415192427995471416716875))/(49517601571415210995964968960000*(1469363753180227599360000*pi + 29915351642033550057827))
Rbsol =
(217954704820530150421311*(4275354068281784437756792116609024*pi + 39038561809674979475803485110635625))/(99035203142830421991929937920000*(1469363753180227599360000*pi + 29915351642033550057827))
My matlab version 2020a

R2021a

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