1又1/3-7/12+9/20-11/30+13/42-15/56+17/72

=1/3-(3+4)/(3*4)+(4+5)/(4*5)-(5+6)/(5*6)+(6+7)/42-(7+8)/(7*8)+(8+9)/(8*9)
=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
=10/9追答

=1又1/3-(3+4)/(3*4)+(4+5)/(4*5)-(5+6)/(5*6)+(6+7)/42-(7+8)/(7*8)+(8+9)/(8*9)
=1又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
=10/9

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