MATLAB: How to replace equations in symbolic function

MATLABreplacevariables

Good Morning,

Please I have a function F = 3*a*b

Therefore how can i change (a*b) with a variable 'x' to obtain F = 3*x

Think you.

Best Answer

By using subs function:

syms a b x
F=3*a*b;
Fnew=subs(F,{a,b},{x,1})

Related Solutions

MATLAB: Problem with simplify function used with matrices

MATLAB's symbolic toolbox is not perfect (perhaps it is impossible to make a perfect symbolic engine), and every year they make some improvements. This seems one such case. Also, "simplification" is not a well defined idea. The tips section of simplify() documentation mentions: "Simplification of mathematical expression is not a clearly defined subject. There is no universal idea as to which form of an expression is simplest. The form of a mathematical expression that is simplest for one problem might be complicated or even unsuitable for another problem."

Are you using an older release of MATLAB? In R2020b, it is simplified as you described in your comment

syms t1 t2 t3 t4 a1 a2 d3
M = [
cos(t4)*(cos(t1)*cos(t2) - sin(t1)*sin(t2)) - sin(t4)*(cos(t1)*sin(t2) + cos(t2)*sin(t1)), - cos(t4)*(cos(t1)*sin(t2) + cos(t2)*sin(t1)) - sin(t4)*(cos(t1)*cos(t2) - sin(t1)*sin(t2)), 0, a1*cos(t1) + a2*cos(t1)*cos(t2) - a2*sin(t1)*sin(t2)
cos(t4)*(cos(t1)*sin(t2) + cos(t2)*sin(t1)) + sin(t4)*(cos(t1)*cos(t2) - sin(t1)*sin(t2)),   cos(t4)*(cos(t1)*cos(t2) - sin(t1)*sin(t2)) - sin(t4)*(cos(t1)*sin(t2) + cos(t2)*sin(t1)), 0, a1*sin(t1) + a2*cos(t1)*sin(t2) + a2*cos(t2)*sin(t1)
                                                                                        0,                                                                                           0, 1,                                                   d3
                                                                                        0,                                                                                           0, 0,                                                   1];
                                                                                    
M_simplify = simplify(M)

Result

>> M_simplify
M_simplify =
[cos(t1 + t2 + t4), -sin(t1 + t2 + t4), 0, a2*cos(t1 + t2) + a1*cos(t1)]
[sin(t1 + t2 + t4),  cos(t1 + t2 + t4), 0, a2*sin(t1 + t2) + a1*sin(t1)]
[                0,                  0, 1,                           d3]
[                0,                  0, 0,                            1]

You may experiment with following option if you are using an older release of MATLAB

M_simplify = simplify(M, 'Seconds', 10); % specify maximum number of seconds MATLAB spent on simplification

MATLAB: How to solve a non-linear equation system as function handle with fsolve

qini = zeros(1,6);             % Initial guess
ci = fsolve(@funec,qini)       % fsolve with the function handle, unknown variables and initial guess
                               % done many variations here, none of them worked
function ec = funec(x);
q1 = x(1);
q2 = x(2);
q3 = x(3);
q4 = x(4);
q5 = x(5);
q6 = x(6);
ec =[...
    sin(q6)*(cos(q1)*cos(q2)*sin(q3)*sin(q4) - cos(q4)*sin(q1) + cos(q1)*cos(q3)*sin(q2)*sin(q4)) - cos(q6)*(cos(q2 + q3)*cos(q1)*sin(q5) + cos(q5)*sin(q1)*sin(q4) + cos(q1)*cos(q2)*cos(q4)*cos(q5)*sin(q3) + cos(q1)*cos(q3)*cos(q4)*cos(q5)*sin(q2)) - 3097002709122497/18014398509481984
    cos(q6)*(cos(q1)*cos(q4) + cos(q2)*sin(q1)*sin(q3)*sin(q4) + cos(q3)*sin(q1)*sin(q2)*sin(q4)) + sin(q6)*(cos(q2 + q3)*sin(q1)*sin(q5) - cos(q1)*cos(q5)*sin(q4) + cos(q2)*cos(q4)*cos(q5)*sin(q1)*sin(q3) + cos(q3)*cos(q4)*cos(q5)*sin(q1)*sin(q2)) - 6124274932042799/36028797018963968
    - sin(q2 + q3)*cos(q5) - cos(q2 + q3)*cos(q4)*sin(q5) - 139701203459709/140737488355328
    70*cos(q1) + 360*cos(q1)*sin(q2) - 65*sin(q1)*sin(q4)*sin(q5) + 380*cos(q1)*cos(q2)*cos(q3) - 380*cos(q1)*sin(q2)*sin(q3) + 65*cos(q1)*cos(q2)*cos(q3)*cos(q5) - 65*cos(q1)*cos(q5)*sin(q2)*sin(q3) - 65*cos(q1)*cos(q2)*cos(q4)*sin(q3)*sin(q5) - 65*cos(q1)*cos(q3)*cos(q4)*sin(q2)*sin(q5) + 19869/50
    sin(q6)*(cos(q1)*cos(q4) + cos(q2)*sin(q1)*sin(q3)*sin(q4) + cos(q3)*sin(q1)*sin(q2)*sin(q4)) - cos(q6)*(cos(q2 + q3)*sin(q1)*sin(q5) - cos(q1)*cos(q5)*sin(q4) + cos(q2)*cos(q4)*cos(q5)*sin(q1)*sin(q3) + cos(q3)*cos(q4)*cos(q5)*sin(q1)*sin(q2)) + 2202239090516653/2251799813685248
    cos(q2 + q3)*cos(q6)*sin(q4) - sin(q6)*(sin(q2 + q3)*sin(q5) - cos(q2 + q3)*cos(q4)*cos(q5)) + 7591943296130745/288230376151711744
    cos(q2 + q3)*cos(q1)*cos(q5) - sin(q5)*(sin(q1)*sin(q4) + cos(q1)*cos(q2)*cos(q4)*sin(q3) + cos(q1)*cos(q3)*cos(q4)*sin(q2)) - 102014544375725/18014398509481984
    70*sin(q1) + 360*sin(q1)*sin(q2) - 380*sin(q1)*sin(q2)*sin(q3) + 380*cos(q2)*cos(q3)*sin(q1) + 65*cos(q1)*sin(q4)*sin(q5) + 65*cos(q2)*cos(q3)*cos(q5)*sin(q1) - 65*cos(q5)*sin(q1)*sin(q2)*sin(q3) - 65*cos(q2)*cos(q4)*sin(q1)*sin(q3)*sin(q5) - 65*cos(q3)*cos(q4)*sin(q1)*sin(q2)*sin(q5) - 1563/20
    cos(q6)*(sin(q2 + q3)*sin(q5) - cos(q2 + q3)*cos(q4)*cos(q5)) + cos(q2 + q3)*sin(q4)*sin(q6) - 2129854249224409/18014398509481984
    cos(q6)*(cos(q1)*cos(q2)*sin(q3)*sin(q4) - cos(q4)*sin(q1) + cos(q1)*cos(q3)*sin(q2)*sin(q4)) + sin(q6)*(cos(q2 + q3)*cos(q1)*sin(q5) + cos(q5)*sin(q1)*sin(q4) + cos(q1)*cos(q2)*cos(q4)*cos(q5)*sin(q3) + cos(q1)*cos(q3)*cos(q4)*cos(q5)*sin(q2)) - 8872946535830847/9007199254740992
    cos(q2 + q3)*cos(q5)*sin(q1) - sin(q5)*(cos(q2)*cos(q4)*sin(q1)*sin(q3) - cos(q1)*sin(q4) + cos(q3)*cos(q4)*sin(q1)*sin(q2)) - 8718732974408059/72057594037927936
    360*cos(q2) - 380*cos(q2)*sin(q3) - 380*cos(q3)*sin(q2) - 65*cos(q2)*cos(q5)*sin(q3) - 65*cos(q3)*cos(q5)*sin(q2) - 65*cos(q2)*cos(q3)*cos(q4)*sin(q5) + 65*cos(q4)*sin(q2)*sin(q3)*sin(q5) - 39571/100];
end

Best Answer

Related Solutions

MATLAB: Problem with simplify function used with matrices

MATLAB: How to solve a non-linear equation system as function handle with fsolve

Related Question