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