将乘积项拆开得到：
原式等于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
1/3*4+1/4*5+1/5*6+1/6*7+1/7*8+1/8*9
=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
拆分1/3*4=1/3-1/4
原式=1/3-1/4+1/4-1/5.....-1/9=2/9
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