Differentiate big symbolic function

I have this code:

if true
  clear all
clc
% a=alpha1; b=alpha2; c=betha
syms a b c N1 N2 N3 N21 N31 n2 n21 n3 l1 l2 l3 P L sa
% Se calculeaza lungimea elementelor
P=2000;
L=1300;
sa=150;
l1=L/cos(a);
l2=L/cos(b);
l3=L/cos(b+c);
% Se calculeaza A
N21=(-P*cos(b+c)/(cos(b)*sin(b+c)-cos(b+c)));
N31=P/(sin(b+c)-cos(b+c)/cos(b));
n31=(-sin(a)*cos(b)-sin(b)*cos(a))/(sin(b+c)*cos(b)-cos(b+c)*sin(b));
n3=n31;
n21=(cos(a)-n3*cos(b+c))/cos(b);
n2=n21;
%efortul din elementul 1
N1=(-((n2*l2*N21+n3*l3*N31)/(l1+n2^2*l2*n3^2*l3)));
%efortul din elementul 3
N3=(P*cos(b)-N1*sin(a)*cos(b)-N1*cos(a))/(cos(b)*(sin(b+c)-cos(b+c)));
%efortul din elementul 2
N2=(N1*cos(a)-cos(b+c)*N3)/(cos(b));
%definirea functiei obiectiv
V1=simple((N1/sa)*l1+(N2/sa)*l2+(N3/sa)*l3);
V=diff(V1(a,b,c),a)
end

I want to diferentiate V1 with respect to a, b and c but i get the following error:

" Error using sym/subsindex (line 1555) Indexing input must be numeric, logical or ':'.

Error in sym>privformat (line 2367) x = subsindex(x)+1;

Error in sym/subsref (line 1575) [inds{k},refs{k}] = privformat(inds{k});

Error in Derivare2 (line 29) diff_f=diff(V1(a,b,c),a) "

What can i do to solve this problem? Thank you

Answer 1

[Walter Roberson]

You want to differentiate V1 but you ask to differentiate V instead, and V is not defined.

[Pavel]

Sorry. I initialy wanted to differentiate V and I got the error message that i posted, but i thought that if i try to simplify V, it will work, but it didn't. So i named V1 to be the simple form of the equation and I named V to be the different form of V1.

[Walter Roberson]

When you build V1 you are applying simple() to an expression, not to a function. Then when you go to integrate you are using V1(a,b,c) which is the syntax for function evaluation (if V1 is a function or function handle) and for indexing otherwise. V1 is a symbolic expression not a function or function handle so indexing is being invoked.

I suspect you want just diff(V1,a)

[Pavel]

Thanks a lot. Matlab differentiated the function. I ended up with three equations with the variables a,b and c. How do i solve for a,b and c? Thanks again

[Walter Roberson]

solve( diff(V1, a), diff(V1, b), diff(V1, c), a, b, c)