 function. We can use this tangent
  line to estimate how much the output ($y$ values) will change if
  we change the input ($x$-value). If we rewrite the notation
  $\ds\frac{dy}{dx}=f'$ in the form $dy=f' dx$, then we can read
  this as &ldquo;A small change in $y$ (called $dy$) equals the
  derivative ($f'$) times a small change in $x$ (called $dx$).&rdquo;</p>

  <div class="definition">
    We call $dx$ the differential of $x$. If $f$ is a function of
    $x$, then the differential of $f$ is $df = f'(x