都是整数相加..,怎么可能出分数 1×2=(1×2×3-0×1×2)÷32×3=(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=30...