1/n(n+1)=1/n-1/(n+1)
原式=1/1×2+1/2×3+1/3×4+1/4×5+1/5×6
=1-1/2+1/2-1/3+1/3-1/4+1/4-1/5+1/5-1/6
=1-1/6
=5/6