1/11*1/13+1/13*1/15++1/15*1/17+1/17*1/19
=(1/2)(2/11*1/13+2/13*1/15+2/15*1/17+2/17*1/19)
=(1/2)(1/11-1/13+1/13-1/15+1/15-1/17+1/17-1/19)
=(1/2)(1/11-1/19)
=4/209
1/11*1/13+1/13*1/15++1/15*1/17+1/17*1/19
=(1/2)(2/11*1/13+2/13*1/15+2/15*1/17+2/17*1/19)
=(1/2)(1/11-1/13+1/13-1/15+1/15-1/17+1/17-1/19)
=(1/2)(1/11-1/19)
=4/209