why do I face problem with matlab solve function?

hi, I'm trying to use the solve function in matlab and I have no idea why I kept getting errors as such:

"??? Error using ==> solve>getEqns at 182 ' L1*cos(pi/2)+L2.*cos(q2)-L3*cos(pi)==d1.*cos(q1) ' is not a valid expression or equation.

Error in ==> solve at 67 [eqns,vars] = getEqns(varargin{:});

Error in ==> test1 at 69 solve('L1*cos(pi/2)+L2.*cos(q2)-L3*cos(pi)==d1.*cos(q1)','L1*sin(pi/2)+L2.*sin(q2)-L3*sin(pi)==d1.*sin(q1)'); "

d1 and q1 are unknowns. L1=800; L2=150; L3=35; q2 is a 10*1 matrix

program:

syms d1 q1

solve('L1*cos(pi/2)+L2.*cos(q2)-L3*cos(pi)==d1.*cos(q1)','L1*sin(pi/2)+L2.*sin(q2)-L3*sin(pi)==d1.*sin(q1)');

Thanks

Answer 1

[Walter Roberson]

When you use a quoted string as the argument to solve(), then you must use MuPAD syntax, which does not recognize "==" and requires "=" instead.

Caution: if you set values for those constants but use quoted strings as the arguments to solve() then the values will not be substituted in. You need to either subs() on the quoted strings or not use quoted strings.

[Qinglin Mok]

sorry i dont understand. i have not studied matlab that in depth. so does it mean that I have to change all the == to =? I tried that and I got error as such:

??? Error using ==> solve>getEqns at 182 ' L1*cos(pi/2)+L2.*cos(q2)-L3*cos(pi)=d1.*cos(q1) ' is not a valid expression or equation.

Error in ==> solve at 67 [eqns,vars] = getEqns(varargin{:});

Error in ==> test1 at 79 solve('L1*cos(pi/2)+L2.*cos(q2)-L3*cos(pi)=d1.*cos(q1)','L1*sin(pi/2)+L2.*sin(q2)-L3*sin(pi)=d1.*sin(q1)'); >>

[Walter Roberson]

solve(subs('L1*cos(pi/2)+L2*cos(q2)-L3*cos(pi)=d1*cos(q1)'), subs('L1*sin(pi/2)+L2*sin(q2)-L3*sin(pi)=d1*sin(q1)'), d1, q1 );

Notice the "." have been removed, and subs() has been added to import the values of the constants.

Alternately

solve( L1*cos(pi/2)+L2.*cos(q2)-L3*cos(pi)==d1.*cos(q1), L1*sin(pi/2)+L2.*sin(q2)-L3*sin(pi)==d1.*sin(q1), d1, q1);

Notice that there are no quotation marks here.

However, this second form will only work from R2011b onwards. Before that you would use

solve(L1*cos(pi/2)+L2.*cos(q2)-L3*cos(pi) - d1.*cos(q1), L1*sin(pi/2)+L2.*sin(q2)-L3*sin(pi) - d1.*sin(q1), d1, q1);

Answer 2

[Roger Stafford]

Your equations are of the form

 d1*cos(q1) = K1
 d1*sin(q1) = K2

for which the solutions can easily be determined without using 'solve'.

 q1 = atan2(K2,K1);
 d1 = sqrt(K1^2+K2^2);

By replacing K1 and K2 by their equivalent in terms of your known parameters you can find all solutions for d1 and q1.

[Qinglin Mok]

hi.. thanks for that alternative solution. I tried but I got a 1x1 sym for d1 and q1 when I'm suppose to get the same size as q2?

syms d1 q1
k1=L1*cos(pi/2)+L2*cos(q2)-L3*cos(pi);
k2=L1*sin(pi/2)+L2*sin(q2)-L3*sin(pi);
k1=d1*cos(q1);
k2=d1*sin(q1);
q1 = atan(k2,k1);
d1 = sqrt(k1^2+k2^2);

[Roger Stafford]

It isn't necessary to declare q1 and d1 as sym. The following will solve for them as vectors the same size as q2.

 K1 = L1*cos(pi/2)+L2*cos(q2)-L3*cos(pi);
 K2 = L1*sin(pi/2)+L2*sin(q2)-L3*sin(pi);
 q1 = atan2(K2,K1);
 d1 = sqrt(K1.^2+K2.^2);