=1/3-1/4+1/4-1/5+1/5-1/6+1/6-1/7+1/7-1/8+1/8-1/9
=1/3-1/9
=2/9 (等于9分之2)
3X(1/4)+4X(1/5)+5X(1/6)+6X(1/7)+7X(1/8)+8X(1/9)
=(3/4)+(4/5)+(5/6)+(6/7)+(7/8)+(8/9)
=(45/60)+(48/60)+(50/60)+(432/504)+(441/504)+(448/504)
=(143/60)+(1321/504)
=(12012/5040)+(13210/5040)
=12611/2520
24