title: How do I assign variables in matrices?
body: <p>I can't make matrices with variables in it for some reason. I get following message.</p>
  
  <pre><code>&gt;&gt;&gt; A= [a b ;(-1-a) (1-b); (1+a) b]
  
  error: horizontal dimensions mismatch (2x3 vs 1x1)
  </code></pre>
  
  <p>Why is it? Please show me correct way if I'm wrong. </p>