=1/2×3+1/3×4+1/4×5+1/5×6+1/6×7+1/7×8
=1/2-1/3+1/3-1/4+1/4-1/5+1/5-1/6+1/6-1/7+1/7-1/8
=1/2-1/8
=3/8