=1/2×(1/5×6-1/6×7)+1/2×(1/6×7-1/7×8)+1/2+(1/7×8-1/8×9)=1/2×(1/5×6-1/6×7+1/6×7-1/7×8+1/7×8-1/8×9)=1/2×(1/5×6-1/8×9)=7/720
7/720