把2提出来得（1/12+1/20+……1/90）*2
把1/12拆成1/3-1/4
同样1/20拆成1/4-1/5
……
得（1/3-1/10)*2=7/15
第二题就=（1-1/3+1/3-1/5+1/5-1/7+1/7-1/9+1/9-1/11）÷2
=5/11
2/12+2/20+2/30+2/42+2/56+2/72+2/90=?1/3+1/15+1/35+1/63+1/99=
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相关问题
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