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