title: SEQUENCE: The general formula
body: <p>Please, can anyone help me get the general formula of the sequence below.</p>
  
  <pre><code>when n=3 the number of arrows is 2
  when n=4 the number of arrows is 4
  when n=5 the number of arrows is 8
  when n=6 the number of arrows is 12
  when n=7 the number of arrows is 18
  when n=8 the number of arrows is 24
  when n=9 the number of arrows is 32 
  when n=10 the number of arrows is 40 etc
  </code></pre>
  
  <p>the n start from 3 upward. </p>
  
  <p>Thank you</p>