1.解答
\begin{eqnarray} (+3)+(-8) &=& (+3) -(+8)\\ &=& -5\\ \\ (-2) + (-5) &=& -(2+5) \\ &=& -7\\ \\ (+7) +(-9) &=& -2\\ \\ (+6) -(-3) &=& (+6)+(+3) \\ &=& 9\\ \\ (+4)+(-2)-(+8) &=& (+4)+(-2)&+&(-8)\\ &=& (+2) &+&(-8)\\ &=& -6\\ \\ 3\color{red}{-5}+6\color{red}{-1}&=& (3+6)&-&(\color{red}{5+1})\\ &=& 9&-&\color{red}{6}\\ &=&3\\ \\ -5-4-2-1&=&-(5+4+2+1)\\ &=&-12 \end{eqnarray}