1/3+1/15+1/35+.+1/99
=1/(1x3)+1/(3x5)+1/(5x7)+……+1/(9x11)
=1/2x(1-1/3+1/3-1/5+1/5-1/7+……+1/9-1/11)
=1/2x(1-1/11)
=1/2x10/11
=5/11