其它
Given: $a-b\textgreater a$ and $a+b\textless b$. Which of the following must be true?~\uline{~~~~~~~~~~}~  $I$. $a b$ is negative  $II$. $a+b$ is negative  $III$. $a-b$ is negative 
Overseas Competition->Knowledge Point->Calculation Modules->Basic Concepts of Equation->Inequalities
$a-b\textgreater a$ then $b\textless0$  $a+b\textless b$ then $a\textless0$.  So both $a$ and $b$ are negative. $I$ is false, since the product will be positive. $II$ is true since the sum of two negative numbers is negative. $III$ is false because if $a=-1$ and $b=-2$ then $a-b$ is positive. Only $II$ is true. 