title: std::pow gives a wrong approximation for fractional exponents
body: <p>Here is what I mean trying to do</p>
  
  <pre><code> double x=1.1402
   double pow=1/3;
   std::pow(x,pow) -1;
  </code></pre>
  
  <p>result is 0 but I expect 0.4465</p>
  
  <p>the equation is  <code>(1 + x) ^3= 1.1402</code>, find <code>x</code>.</p>