都是整数相加..,怎么可能出分数
1×2=(1×2×3-0×1×2)÷3
2×3=(2×3×4-1×2×3)÷3
...
20×21=(20×21×22-19×20×21)÷3
原式=(1×2×3-0×1×2+2×3×4-1×2×3+...+20×21×22-19×20×21)÷3
=20×21×22÷3
=3080
都是整数相加..,怎么可能出分数
1×2=(1×2×3-0×1×2)÷3
2×3=(2×3×4-1×2×3)÷3
...
20×21=(20×21×22-19×20×21)÷3
原式=(1×2×3-0×1×2+2×3×4-1×2×3+...+20×21×22-19×20×21)÷3
=20×21×22÷3
=3080