title: MATLAB vpa() doesn't compute variable-point number for expression with exponent?
body: <p>Trying to use vpa() to compute a variable point number for a rational expression in an exponent:</p>
  
  <pre><code>syms x;
  ans1 = x^(12345/67890)
  ans2 = vpa(x^(12345/67890),3)
  ans2_5 = vpa((12345/67890),3)
  ans3 = vpa(x*(12345/67890),3)
  </code></pre>
  
  <p>The above shows the issue. ans1 shows the default output of the expression. ans2 shows that vpa() is not computing the variable point number for the expression. ans 2_5 shows what it should be computing to. The result I'm looking for is <code>x^0.182</code>.</p>
  
  <p>ans3 just shows that vpa() produces the expected result when the function is multiplication--it's something in the exponent that's tripping it up.</p>
  
  <p>How can I request that the exponent be evaluated by vpa?</p>
  
  <p>[edit]</p>
  
  <p>Maybe I can make this more clear. All I really need is an accessor or index to the exponent of an exponential expression. So if my expression is 
  <code>y = x^a</code>
  I need to be able to have some accessor on <code>x</code> that returns <code>a</code>.</p>
  
  <p>Is this possible?</p>