有一列数a₁，a₂，a₃，…，a_n，满足下列条件：a₁=0，|a₂|=|a₁+1|，|a₃|=|a₂+1|，…，|a_n|=|a_n﹣1+1|．

求证：a₁，a₂，a₃，…，a_n这n个数的算术平均数不小于-$\frac{1}{2}$．

$\frac{6}{7}$ ÷7=____      $\frac{1}{9}\times 3$ =____     1+50%=____    $\frac{4}{5}\times 15$ =____

$\frac{3}{4}\times \frac{1}{5}$ ÷ $\frac{3}{4}\times \frac{1}{5}$ =____    $\frac{1}{6}$ + $\frac{5}{6}$ × $\frac{1}{5}$ =____   $\frac{1}{2}$ ×101－ $\frac{1}{2}$ =____   $\frac{3}{4}$ ×9× $\frac{4}{3}$ =____

The doctor ________the boy as soon as he was sent to the hospital.