1/6+1/12+1/20+1/30+1/42
=1/(2×3)+1/(3×4)+1/(4×5)+1/(5×6)+1/(6×7)
=1/2-1/3+1/3-1/4+1/4-1/5+1/5-1/6+1/6-1/7
=1/2 -1/7
=5/14