1/1×2+1/2×3+1/3×4+1/4×5+...+1/39×40
问题描述：
1/1×2+1/2×3+1/3×4+1/4×5+...+1/39×40
答
1/1×2+1/2×3+1/3×4+1/4×5+...+1/39×40
=1/1-1/2+1/2-1/3+1/3-1/4+1/4-1/5+...+1/39-1/40
=1-1/40
=39/40