title: error:'isolate' undefined near line 7 column 7
body: <p>I have a PROBLEM WITH THIS CODE exactly in the line 7 (isolate).Please someone can read this code and help me to fix the problem.Zhen I run my code they tell me that there's an error in the line 7 column 7.</p>
  <pre><code>eqn = m*a == -m*g*sin(theta);
  
  syms r
  eqn = subs(eqn,a,r*diff(theta,2));
  
  eqn = isolate(eqn,diff(theta,2));
  
  syms omega_0
  eqn = subs(eqn,g/r,omega_0^2);
  
  syms x
  approx = taylor(sin(x),x,'Order',2);
  approx = subs(approx,x,theta(t));
  
  eqnLinear = subs(eqn,sin(theta(t)),approx);
  
  syms theta_0 theta_t0
  theta_t = diff(theta);
  cond = [theta(0) == theta_0, theta_t(0) == theta_t0];
  assume(omega_0,'real')
  thetaSol(t) = dsolve(eqnLinear,cond);
  
  gValue = 9.81;
  rValue = 1;
  omega_0Value = sqrt(gValue/rValue);
  T = 2*pi/omega_0Value;
  
  theta_0Value  = 0.1*pi; % Solution only valid for small angles.
  theta_t0Value = 0;      % Initially at rest. 
  
  vars   = [omega_0      theta_0      theta_t0];
  values = [omega_0Value theta_0Value theta_t0Value];
  thetaSolPlot = subs(thetaSol,vars,values);
  
  fplot(thetaSolPlot(t*T)/pi, [0 5]);
  grid on;
  title('Pendulum Motion');
  xlabel('t/T');
  ylabel('\theta/\pi'); 
  
  x_pos = sin(thetaSolPlot);
  y_pos = -cos(thetaSolPlot);
  fanimator(@fplot,x_pos,y_pos,'ko','MarkerFaceColor','k','AnimationRange',[0 5*T]);
  hold on;
  fanimator(@(t) plot([0 x_pos(t)],[0 y_pos(t)],'k-'),'AnimationRange',[0 5*T]);
  fanimator(@(t) text(-0.3,0.3,&quot;Timer: &quot;+num2str(t,2)+&quot; s&quot;),'AnimationRange',[0 5*T]); ```
  </code></pre>