title: Sum column every n column in a data frame R
body: <p>I have a <code>df(A)</code> with 10 column and 300 row. I need to sum every two column, between them, in this way: </p>
  
  <pre><code>A[,1]+A[,2] = # first result
  A[,3]+A[,4] = # second result
  A[,5]+A[,6]= # third result
  ....
  A[,9]+A[,10] # last result
  </code></pre>
  
  <p>The expected final result is a new dataframe with 5 column and 300 row.
  Any way to do this? with <code>tapply</code> or loop <code>for</code>? 
  I know that i can try with the upon example, but i'm looking for a fast method
  Thank you</p>