1／2×3＋1／3×4＋1／4×5······﹢1／49×50
=1/2-1/3 + 1/3-1/4 + 1/4-1/5+...+1/49-1/50
=1/2-1/50
=12/25不用谢哦 用裂项想减法解决~哦，我采纳你