Cannot understand the error in my code to solve differential equation. Please someone tell me the corrections to be made.

clf

syms t m

ycf='exp(m*t)';clf

syms t m

ycf='exp(m*t)';

y1=diff(ycf,t);

y2=diff(y1,t);

y3=simplify(subs('D2y+y=0',{'y','Dy','D2y'},{ycf,y1,y2}));

ycf=solve(y3,'m');

y4='(A*cos(2*t))';

y5=diff(y4,t);

y6=diff(y5,t);

y7=simplify(subs('D2y+y=cos(2*t)',{'y','Dy','D2y'},{y4,y5,y6}));

ypi=solve('y7','A');

ERROR-

Error using sym>convertChar (line 1448)

Character vectors and strings in the first argument can only specify a variable or number. To evaluate

character vectors and strings representing symbolic expressions, use 'str2sym'.

Error in sym>tomupad (line 1214)

S = convertChar(x);

Error in sym (line 211)

S.s = tomupad(x);

Error in sym/privResolveArgs (line 966)

argout{k} = sym(arg);

Error in sym/diff (line 21)

args = privResolveArgs(S,varargin{:});

Error in qwedfgh (line 4)

y1=diff(ycf,t);

Answer 1

[Navya Singam]

Hi Mehul,

The mentioned error in the code is because of the following lines

ycf = 'exp(m*t)'
y1 = diff(ycf,t);

ycf is of the type 'char', but "diff" expects the first argument to be of type symbolic expression or symbolic function. To convert the char into sym, "str2sym" function can be used.

ycf = str2sym('exp(m*t)')
y1 = diff(ycf,t)

Alternatively, you may also define the variable ycf as

ycf = exp(m*t)

since, exp(m*t) is a symbolic expression.

Refer to the str2sym and diff documentation to know more about the input arguments to the "diff" function.