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