原式=1/2+1/(2×3)+1/(3×4)+1/(4×5)+.+1/(9×10)
=1/2+1/2-1/3+1/3-1/4+1/4-1/5+.1/9-1/10
=1-1/10
=9/10
规律是1/n-1/(n+1)=1/n(n+1)