1/(1×2)+1/(2×3)+1/(3×4)+...+1/(8×9)+1/(9×10)=1-1/2+1/2-1/3+1/3-1/4+...+1/8-1/9+1/9-1/10=1-1/10=9/10
1/1×2＋1/2×3＋1/3×4＋……1/8×9＋1/9×10=1-1/2+1/2-1/3+1/3-1/4+......+1/9-1/10=1-1/10=9/10
