Why am I getting this error? The code works to solve the equation in terms of a and b for my teacher in class, but my professor is not having the issue. It solves for her.

CODE:

syms a b x
solve('a*x^2 + b*x','b')

ERROR:

Error using solve>getEqns (line 418)
List of equations must not be empty.

Error in solve (line 226)
[eqns,vars,options] = getEqns(varargin{:});

Error in Untitled (line 63)
solve('a*x^2 + b*x','b')

Answer 1

[Star Strider]

Lose the single quotes and it will solve for you, too:

syms a b x
B = solve(a*x^2 + b*x, b)

B =

-a*x

[Walter Roberson]

The form of solve() that permits a character vector of equation is not valid in R2018a and later.

[MURRAY GILBERTSON]

So what does that mean exactly?

I downloaded 2018a today and now my code for the 2017b is no longer working.

G=solve('(M1*d^2+C1*d+K1)*Z=(C1*d+K1)*W','(M2*d^2+C1*d+C2*d+K1+K2)*W=(C1*d+K1)*Z+(C2*d+K2)*V','(M3*d^2+C2*d+C3*d+K2+K3)*V=(C2*d+K2)*W+(C3*d+K3)*U','Z,V,W')

[Star Strider]

String expressions no longer work in the Symbolic Math Toolbox in R2018a and later versions.

Please read the Release Notes (link).

[MURRAY GILBERTSON]

Character vectors as inputs to solve Errors

When specifying equations and variables, use symbolic equations and variables instead of character vectors. For example, replace solve('2*r = 1', 'r') with syms r; solve(2*r == 1, r).

Replace all instances of character vector input. Instead, create symbolic variables using syms, and then pass them as comma-separated input arguments, or as a vector of input arguments.

[Star Strider]

Q.E.D.