1/6+1/30+1/70+1/126+1/198
=1/4*(2/3+2/15+2/35+2/63+2/99)
=1/4*(1-1/3+1/3-1/5+1/5-1/7+1/7-1/9+1/9-1/11)
=1/4*(1-1/11)
=1/4*10/11
=5/22