第13页 - 江苏版实验班提优训练数学七年级上下册答案

$\frac {1}{2}×(\frac {1}{2021}-\frac {1}{2023})$

解：原式$ =\frac {1}{2} × (1-\frac {1}{3})+\frac {1}{2} ×( \frac {1}{3}-\frac {1}{5})+\frac {1}{2} ×(\frac {1}{5}-\frac {1}{7})$

$+···+\frac {1}{2}×(\frac {1}{2021}-\frac {1}{2023})$

$=\frac {1}{2}× (1-\frac {1}{3}+\frac {1}{3}-\frac {1}{5}+\frac {1}{5}-\frac {1}{7}+···+\frac {1}{2021}-\frac {1}{2023})$

$=\frac {1}{2}×(1-\frac {1}{2023})$

$=\frac {1}{2}×\frac {2022}{2023}$

$= \frac {1011}{2023}$

解：$(1)$小军的解法较好

$(2)49 \frac {24}{25}×(-5)$

$=(50-\frac {1}{25})×(-5)$

$=50×(-5)-\frac {1}{25}×(-5)$

$=-250+\frac {1}{5}$

$=-249 \frac {4}{5}$

$(3)9 \frac {5}{6}×(-3)$

$=(10-\frac {1}{6})×(-3)$

$=10×(-3)-\frac {1}{6}×(-3)$

$=-30+\frac {1}{2}$

$=-29 \frac {1}{2}$