原式=1/2-(1/2×3+1/3×4+1/4×5+1/5×6+1/6×7+1/7×8)=1/2-(1/2-1/3+1/3-1/4+1/4-1/5+1/5×1/6+1/6-1/7+1/7-1/8)=1/2-(1/2-1/8)=1/2-1/2+1/8=1/8
=（1/1-1/2)-(1/2-1/3)-(1/3-1/4)-......-(1/7-1/8)=1-1+1/8=1/8
