title: How to add columns according to element-wise multiplications
body: <p>I have a table that contains two columns of numbers. I am trying to generate a new table where the result of each column comes from the element wise multiplication of the previous <code>data.frame</code> columns 
  For example, i have this:</p>
  
  <pre><code>df = data.frame(A=c(2,5,3), B=c(3,2,4))
  print(df)
  
    A B
  1 2 3
  2 5 2
  3 3 4
  </code></pre>
  
  <p>And i need :</p>
  
  <pre><code>        3  2   4
      2   6  4   8
      5  15 10  20
      3   9  6  12
  </code></pre>