1/2+2/2×3+3/2×3×4+4/2×3×4×5+5/2×3×4×5×6+6/2×3×4×5×6×7=
热心网友
原式:=1/2+1/3+1/8+1/30+1/144+1/840 =72/144+18/144+1/144+280/840+28/840+1/840 =91/144+309/840 =5039/5040
热心网友
原式=（2-1）/2+（3-1）/2×3+（4-1）/2×3×4+…+(7-1)/2×3×4×5×6×7=(1-1/2) +(1/2-1/2×3)+(1/2×3-1/2×3×4)+…+(1/2×3×4×5×6-1/2×3×4×5×6×7)=1-1/2×3×4×5×6×7=5039/5040