My code fails to get value for M (M and K should be very small numbers, close to zero I believe).

Here is my code, it works, but it doesn't give back the right value for M. It has worked for other problems, maybe something is different, or maybe my "g" should be different?

syms x K M c1 c2 c3 c4
x0 = [0,0,0,0];
y0 = [1,200,-80,900];
coeff = [1,-0.02,12.5,-0.058,27.77];
rx = -0.05*exp(0.005*x)*cos(11.7*x) + 0.07*exp(0.005*x)*sin(11.7*x);
y_new = Examprac(x0,y0,coeff,rx);

then the function is here below:

function y = Examprac(x0,y0,coeff,rx)
    syms x c1 c2 c3 c4 K M
    yp = 2*K*cos((117*x)/10)*exp(x/200) + 2*M*sin((117*x)/10)*exp(x/200);
    yp_der1 = diff(yp);
    yp_der2 = diff(yp_der1);
    yp_der3 = diff(yp_der2);
    yp_der4 = diff(yp_der3);
    g = coeff(1)*yp_der4 + coeff(2)*yp_der3 + coeff(3)*yp_der2 + coeff(4)*yp_der1 + coeff(5)*yp == rx;
    C = solve(g, [M, K])
    M_value = double(C.M);
    K_value = double(C.K);
    C = [M_value, K_value]
    yp = subs(yp,K,C(2))
    yp = subs(yp,M,C(1))
    y = yp;
end

Answer 1

[Sam Chak]

Hi @Sean Rotmansky

You have one equation, 'g', but you are attempting to solve for two unknown variables, M and K. The system is considered underdetermined.

% Equation g

g = coeff(1)*yp_der4 + coeff(2)*yp_der3 + coeff(3)*yp_der2 + coeff(4)*yp_der1 + coeff(5)*yp == rx;

% solve g for M and K

C = solve(g, [M, K])