1. 计算:
(1) $-6 + 9 + 6$;
(2) $1\dfrac{3}{4} - (+6) - 3\dfrac{5}{8} + (-1.75) - (-3\dfrac{5}{8})$.

(1) 解:原式=(-6+6)+9=9.
(2) 解:原式=$[1\dfrac{3}{4}+(-1.75)]-6+(3\dfrac{5}{8}-3\dfrac{5}{8})=-6$.

2.计算：
(1)$-7+3-6+2+8$；
(2)$(+9)-(+10)+(-2)-(-8)+3$.

(1) 解:原式=(-7-6)+(3+2+8)=(-13)+13=0.
(2) 解:原式=9-10-2+8+3=(9+8+3)+(-10-2)=20-12=8.

3.计算：
(1)$3.5 - 4.6 + 3.5 - 2.4$；
(2)$|0.75| + (-3) - (-0.25) + \left| -\dfrac{1}{8} \right| + \dfrac{7}{8}$。

(1) 解:原式=(3.5+3.5)+(-2.4-4.6)=7+(-7)=0.
(2) 解:原式=$0.75-3+0.25+\dfrac{1}{8}+\dfrac{7}{8}=(0.75+0.25)+(\dfrac{1}{8}+\dfrac{7}{8})-3=1+1-3=-1$.

4. 计算：
(1) $\frac{4}{5} + \frac{4}{17} + \frac{1}{5} - \frac{1}{17}$；
(2) $-21\frac{2}{3} + (+3\frac{1}{4}) - (-\frac{2}{3}) - (+\frac{1}{4})$；
(3) $-4\frac{7}{8} - (-5\frac{1}{2}) + (-4\frac{1}{2}) - 3\frac{1}{8}$；

(1) 解:原式=$(\dfrac{4}{5}+\dfrac{1}{5})+(\dfrac{4}{17}-\dfrac{1}{17})=1+\dfrac{3}{17}=1\dfrac{3}{17}$.
(2) 解:原式=$(-21\dfrac{2}{3}+\dfrac{2}{3})+(3\dfrac{1}{4}-\dfrac{1}{4})=-21+3=-18$.
(3) 解:原式=$-4\dfrac{7}{8}+5\dfrac{1}{2}-4\dfrac{1}{2}-3\dfrac{1}{8}=(-4\dfrac{7}{8}-3\dfrac{1}{8})+(5\dfrac{1}{2}-4\dfrac{1}{2})=-8+1=-7$.