1/3+1/15+1/35+1/63+...+1/9999
=1/2*(1-1/3)+1/2*(1/3-1/5)+.+1/2*(1/99-1/101)
=1/2*(1-1/3+1/3-1/5+1/5-1/7+.+1/99-1/101)
=1/2*(1-1/101)
=1/2*100/101
=50/101