1/3*5=( 1/3- 1/5)/2

.

.

.

.

1/ 17* 19=( 1/ 17- 1/ 19)/2

So the original formula = {1-1/3+1/3-1/5+1/7+1/65438.

=( 1- 1/ 19)/2

=( 18/ 19)/2

=9/ 19