how i can solve Error using sym/subsindex (line 1558) Indexing input must be numeric, logical or ':'. Error in sym>privformat (line 2357) x = subsindex(x)+1; Error in sym/subsref (line 1578) [inds{k},refs{k}] = privformat(inds{k});

syms y x fn
fn=cos(x)
b=y+2*y^2
y=fn(b)

Answer 1

[Walter Roberson]

Your fn is a scalar symbolic variable. The only valid indices you can use for it are 1 or true or false. Your fn is not a function. To define a function you would have needed

fn(x) = cos(x)

With sufficiently old MATLAB releases you would have needed

fn = symfun(cos(x), x)

[sadeem alqarni]

thanks , how i can substitution y=fn(b)

[Walter Roberson]

syms y x fn
fn(x) = cos(x);
b = y + 2*y^2;
z = fn(b);