title: Use awk to calculate sum for multiple columns
body: <p>I have a list with 8 columns, in which the first 6 are the same. </p>
  
  <pre><code>6   99999715    99999771    NM_001013399    0   -   23  0.0714286
  6   99999715    99999771    NM_001013399    0   -   24  0.0178571
  6   99999715    99999771    NM_001013399    0   -   25  0.1250000
  </code></pre>
  
  <p>I need to calculate the average for column 7 and column8, as well as $7*$8, and get the format like:</p>
  
  <pre><code>6   99999715    99999771    NM_001013399    0   -   ave($7) ave($8) ave($7*$8)
  </code></pre>
  
  <p>How should I do this?
  thx</p>