title: How can I evaluate the matrix in x0=(0,0) if the first variable x doesn't appear?
body: <p>I made a program for the "Newton-Raphson method for vectorial function".</p>
  
  <p>Assume I have the function <code>f(x,y)=(x-y,x+y^2)</code>. The Jacobian matrix is <code>[1,-1;1,2*y]</code>. Then I put <code>r=symvar(jacobian)</code> and to evaluate it, I use <code>subs(jacobian,r,x0)</code>.</p>
  
  <p>How can I evaluate the matrix in <code>x0=(0,0)</code> if the first variable <code>x</code> doesn't appear?</p>