1 / (5x7) +1 / (7x9) +1 / (9x11)+ 1 / (11x13)+ 1 / (13x15)
=1/2×(1/5-1/7+1/7-1/9+1/9-1/11+1/11-1/13+1/13-1/15)
=1/2×(1/5-1/15)
=1/15