1/35 +1/63 +1/99 +1/143 +……+1/575=1/2×(1/5-1/7+1/7-1/9+1/9-1/11+1/11-1/13+……+1/23-1/25)=1/2×(1/5-1/25)=1/2×4/25=2/25
