∴ 1/ 1( 1+ 1)= 1- 1/2

1/2(2+ 1)= 1/2- 1/3

1/3(3+ 1)= 1/3- 1/4

………………

1/20 12(20 12+ 1)= 1/20 12- 1/20 13

Add up the above 20 12 equations, and notice that the second term in the previous row on the right side of each equation cancels the first term in the next row, and you get

The original summation formula =1-1/2013 = 2012/2013.

Is that your understanding? Do it yourself, I believe you can do it!