1/1×2+1/2×3=2/3
1/1×2+1/2×3+1/3×4=3/4
1/1×2+1/2×3+1/3×4+1/4×5=4/5
1/1×2+1/2×3+1/3×4+1/4×5+1/5×6=5/6
自己算的,我初三了,这种题应该不会错.