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第79页

信息发布者：

$=(1+3+5+.….+15+17)+(\frac {1}{6}+ \frac {1}{12}+ \frac {1}{20}+ \frac {1}{72}+\frac {1}{90})$

$=81+(\frac {1}{2}-\frac {1}{3}+\frac {1}{3}-\frac {1}{4}+\frac {1}{4}-\frac {1}{5}+.….+\frac {1}{8}−\frac {1}{9}+\frac {1}{9}−\frac {1}{10})$

$=81+(\frac {1}{2}-\frac {1}{10})$

$=81+\frac {2}{5}$

$=81\frac {2}{5}$

$=1−\frac {1}{2}+1−\frac {1}{6}+1− \frac {1}{12} +.…. + 1-\frac {1}{90}$

$= 1 × 9−\frac {1}{2}+\frac {1}{6}+\frac {1}{12}+.….+\frac {1}{90})$

$=9−(1−\frac {1}{2}+\frac {1}{2}−\frac {1}{3}+\frac {1}{3}−\frac {1}{4}+...+\frac {1}{9}−\frac {1}{10})$

$=9−(1−\frac {1}{10})$

$=9−\frac {9}{10}$

$=8\frac {1}{10}$

$=(1+1+1+1+1+1+11)−(\frac {1}{2}+\frac {1}{4}+\frac {1}{8}$

$+\frac {1}{16}+\frac {1}{32}+\frac {1}{64}+\frac {1}{128})$

$=7−(1−\frac {1}{2}+\frac {1}{2}−\frac {1}{4}+\frac {1}{4}−\frac {1}{8}+\frac {1}{8}−\frac {1}{16}+\frac {1}{16}−\frac {1}{32}$

$+\frac {1}{32}−\frac {1}{64}+\frac {1}{64}−\frac {1}{128})$

$=7−(1−\frac {1}{128})$

$=7−\frac {127}{128}$

$=6\frac {1}{128}$

$2$

$3$

$9$

$60$

$30$

$20$

答:去掉$\frac{1}{12}$和$\frac{1}{15}$这两个分数，才能使余下的分数之和为1。

答：那么△与○的和是3。