第79页 - 江苏版实验班提优训练数学五年级上下册答案

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

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

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

$=81\frac 25$

$=(1-\frac {1}{2})+(1-\frac {1}{6})+(1-\frac {1}{12})+...+(1-\frac {1}{90})$

$=(1+1+...+1)-(\frac {1}{2}+\frac {1}{6}+\frac {1}{12}+...+\frac {1}{90})$

$=9-\frac {9}{10}$

$=8\frac 1{10}$

$=1-\frac {1}{2}+1-\frac {1}{4}+1-\frac {7}{8}+...+1-\frac {1}{128}$

$=7-(1-\frac {1}{128})$

$=6\frac 1{128}$

$=1-\frac {1}{4}+\frac {1}{8}-\frac {1}{8}+\frac {1}{16}-\frac {1}{16}+\frac {1}{32}-\frac {1}{32}+\frac {1}{64}$

$=1-\frac {1}{4}+\frac {1}{64}$

$=\frac {49}{64}$

$\frac {3}{5}$ + $\frac {7}{10}$ -1=$\frac {3}{10}$

答：获二等奖的人数占获奖总人数的$\frac {3}{10}。$

2

3

9

60

30

20

答：去掉$\frac {1}{12}$和$\frac {1}{15}。$

原式$=\frac {3△}{33}+\frac {11○}{33}$

所以$3△+11○=17$

因为△和○表示不同的自然数，所以$△=2，$$○=1$

所以△和○的和是$3。$