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