# Solving a matrix equation with a parameter in the coefficient matrix

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Tuur Vanderhaegen on 26 Nov 2020
Commented: Tuur Vanderhaegen on 27 Nov 2020
Hello,
I'm asked to give an expression of S and F in function of theta. This is possible due to 3 given equations. If I make a matrix of these equations and use the linsolve there is an error because of the parameter thetea. Can someone help?
Kind regards,
Tuur
syms theta S F
r = 0.025;
h = 0.3;
rho = 7850;
g = 9.81;
m = r^2 * pi * h * rho;
phi = atan((0.15 - 0.15*sin(theta))/(0.15*cos(theta)))
F_g = [0 , -m * g];
F_s = [-S*cos(phi), S*sin(phi)];
F_p = [F*cos(theta), F*sin(theta)];
eq1 = F_g(1) + F_s(1) + F_p(1) == 0;
eq2 = F_g(2) + F_s(2) + F_p(2) == 0;
eq3 = 0.15*cos(theta)*F*sin(phi) - 0.15*sin(theta)*F*cos(phi) + m * g * 0.15 * cos(theta) == 0;
A = [-cos(phi), cos(theta); sin(phi), sin(theta); 0, 0.15*cos(theta)*sin(phi) - 0.15*sin(theta)*cos(phi)]
B = [0; m * g ; -m * g * 0.15 * cos(theta)]

Stephan on 27 Nov 2020
syms theta S F
r = 0.025;
h = 0.3;
rho = 7850;
g = 9.81;
m = r^2 * pi * h * rho;
phi = atan((0.15 - 0.15*sin(theta))/(0.15*cos(theta)));
F_g = [0 , -m * g];
F_s = [-S*cos(phi), S*sin(phi)];
F_p = [F*cos(theta), F*sin(theta)];
eq(1) = F_g(1) + F_s(1) + F_p(1) == 0;
eq(2) = F_g(2) + F_s(2) + F_p(2) == 0;
eq(3) = 0.15*cos(theta)*F*sin(phi) - 0.15*sin(theta)*F*cos(phi) + m * g * 0.15 * cos(theta) == 0;
% New equations
eqn(1) = lhs(eq(1)) == lhs(eq(3));
eqn(2) = lhs(eq(2)) == lhs(eq(3));
sol = solve(eqn,F,S)
pretty(sol.F)
pretty(sol.S)

#### 1 Comment

Tuur Vanderhaegen on 27 Nov 2020
Thank you very much :)