# Warning: The solutions are parameterized by the symbols: z

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Ross Hanna on 18 Apr 2017
Commented: Andrew Newell on 20 Apr 2017
Hi There
This is my first time posting, as i am new to matlab, so apologies if i have missed some etiquette that i know there is on some forums.
i am trying to calculate some values for angles of a mechanical arm with a linkage system. i have drawn out the system and found all the relationships i possibly can using the cosine rule and put them all into the file, hence some are not applicable to this bit of code.
my problem is i am trying to use the solve function to solve the following equation for zeta;
k2=sqrt(s^2+h^2-2*s*h*cos(theta+zeta))
i can find the solution of this using wolframalpha as;
zeta=-acos((h^2-k^2+s^2)/(2*h*s))-t
however if i write solve(k2, zeta) it comes up with the following error;
Warning: The solutions are parameterized by the symbols: z7.
To include parameters and conditions in the solution, specify the 'ReturnConditions' option.
> In solve>warnIfParams (line 500)
In solve (line 356)
Warning: The solutions are valid under the following conditions: (in((theta + z7 + acos((H^2 + 270400)/(1040*H)))/(2*pi), 'integer') | in((theta + z7 - acos((H^2 + 270400)/(1040*H)))/(2*pi), 'integer')) & -1 <= (H^2 +
270400)/(1040*H) & (H^2 + 270400)/(1040*H) <= 1 & 0 < z7.
To include parameters and conditions in the solution, specify the 'ReturnConditions' option.
> In solve>warnIfParams (line 507)
In solve (line 356)
I know that the answer for theta and zeta are 0.271 and 0.757 at the point where l is 330, i could just write in what the rearrangement is but that kind of defeats the object of having a maths package in my eyes and i would like to know for future knowledge what i'm doing wrong.
Hopefully someone can help me by telling me i'm an idiot and i've missed something stupid.
Ross
Ross Hanna on 19 Apr 2017
ok, so i've got past the first problem which is getting matlab to give me an answer other than "z". i did this by making theta and zeta "real" when writing syms. the problem now is its giving me two answers, one positive, one negative. i have tried B=A(A=>0) but it doesnt work. i have tried "assume" but it comes up with the error message
Undefined function 'assume' for input arguments of type 'logical'
any help would be greatly appreciated. thanks

Andrew Newell on 19 Apr 2017
The function solve is part of the Symbolic Math Toolbox. Are you defining the symbolic variables? If I run this code
syms s h theta zeta
k2=sqrt(s^2+h^2-2*s*h*cos(theta+zeta));
zeta = solve(k2, zeta)
I get
zeta =
- theta - acos((h^2 + s^2)/(2*h*s))
acos((h^2 + s^2)/(2*h*s)) - theta
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Andrew Newell on 20 Apr 2017
If this problem is well formulated, you could try both values of zeta, one at a time, and then determine whether one of them can be ruled out by the mechanical constraints. If not, you may need to think about the physics some more!