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原式=(7-6+5-4+3-2+1)+(5/6-7/12+9/20-11/30+13/42-25/56+17/72)
=4+[(1/2+1/3)-(1/3+1/4)+(1/4+1/5)-(1/5+1/6)+(1/6+1/7)-(1/7+1/8)+(1/8+1/9)]
=4+(1/2+1/3-1/3-1/4+1/4+1/5-1/5-1/6+1/6+1/7-1/7-1/8+1/8+1/9)
=4+(1/2+1/9)
=4又18分之11
=4+[(1/2+1/3)-(1/3+1/4)+(1/4+1/5)-(1/5+1/6)+(1/6+1/7)-(1/7+1/8)+(1/8+1/9)]
=4+(1/2+1/3-1/3-1/4+1/4+1/5-1/5-1/6+1/6+1/7-1/7-1/8+1/8+1/9)
=4+(1/2+1/9)
=4又18分之11
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