solve unknown constants A, B and C

%% I try to calculate the three unknown constants A,B, and C based on the 1, 2, and 3 equation that I marked by pencil
%% But There is an error message indicates that "Unrecognized function or variable 'Mxy'"
%% How can I solve the three equations, if there is any suggestion, please feel free to let me know!
%% Thank you very much!!
clc
clear
syms A B C 
n=3;
y=1;
a=600;
h=15;
v=0.3;
beta=sqrt((n^2*pi^2/(a^2))+(10/h^2));


eqns=[Mxy==-A*(1-v)+B*(1-v+(2/5)*(n^2*pi^2*h^2)/(a^2))+C*((a^2/(n^2*pi^2))+(h^2/5))*(n^2*pi^2/a^2)*cos(n*pi*y/a),Mx==(-A*(1-v)+2*B*(1+((n^2*pi^2*h^2)/(5*a^2)))+((C*beta*a*h^2)/(5*n*pi)))*(n^2*pi^2/a^2)*sin(n*pi*y),...
    Qx==-(2*B*(n*pi/a)^3+C*(n*pi/a))*sin(n*pi*y/a)];
S=solve(eqns,A,B,C)
A=S.A
B=S.B
C=S.C

Answer 1

[Star Strider]

Define ‘Mxy‘, ‘Mx’, and ‘Qx’ in the syms declaration if no values ared defined for them.

%% I try to calculate the three unknown constants A,B, and C based on the 1, 2, and 3 equation that I marked by pencil
%% But There is an error message indicates that "Unrecognized function or variable 'Mxy'"
%% How can I solve the three equations, if there is any suggestion, please feel free to let me know!
%% Thank you very much!!
% clc
% clear
syms A B C Mxy Mx Qx 
n=3;
y=1;
a=600;
h=15;
v=0.3;
beta=sqrt((n^2*pi^2/(a^2))+(10/h^2));


eqns=[Mxy==-A*(1-v)+B*(1-v+(2/5)*(n^2*pi^2*h^2)/(a^2))+C*((a^2/(n^2*pi^2))+(h^2/5))*(n^2*pi^2/a^2)*cos(n*pi*y/a),Mx==(-A*(1-v)+2*B*(1+((n^2*pi^2*h^2)/(5*a^2)))+((C*beta*a*h^2)/(5*n*pi)))*(n^2*pi^2/a^2)*sin(n*pi*y),...
    Qx==-(2*B*(n*pi/a)^3+C*(n*pi/a))*sin(n*pi*y/a)];
S = solve(eqns,A,B,C);

S = struct with fields:
    A: (1511157274518286468382720*(2525139333831674350099986009185250206368550689820477775499277674242237931066555052851200000*Mxy + 14978046062318810718950739371470519539853358944686543437555228931685540328897460944712499200000000*Qx - 22284748388481…
    B: (42535295865117307932921825928971026432*(73680728475320241462839948936895258395643221076945313747333676322247465028812800000*Qx - 39150960164355549520141067478690196535862474302259608824270739256313334994836925*pi*Qx + 3355062614714504164538684…
    C: -(2283596308329535809693257551119192218212394598400*pi*(968836245234758528816644797919619394171374788137247490470379192066206931473137664*Mx - 87825825062379193201793184198736871618566259588574297335278109*Mxy + 93772165783889616735951215746508…

A = vpa(S.A, 5)

A = 

B = vpa(S.B, 5)

B = 

C = vpa(S.C, 5)

C = 

.

[Mark]

Thanks you very much! Star Strider

[Star Strider]

As always, my pleasure!