1/3+1/15+1/35+1/63+1/99+1/143=1/2x(1-1/3)+1/2x(1/3-1/5)+.....1/2x(1/9-1/11)+1/2(1/11-1/13)=1/2x(1-1/3+1/3-1/5+....+1/9-1/11+1/11-1/13)=1/2x(1-1/13)=1/2x12/13=6/13