3. 把下列各式先写成省略括号的形式，再计算：
(1) $-8-(-15)+(-9)-(-12)$；
(2) $(-\dfrac{6}{5})-7-(-3.2)+(-1)$；
(3) $(-11\dfrac{2}{3})-(-7\dfrac{2}{5})-12\dfrac{1}{3}-(-4.2)$；
(4) $2\dfrac{3}{5}+(-1\dfrac{1}{2})+3\dfrac{3}{10}-(-2\dfrac{1}{2})$；
(5) $\dfrac{3}{4}+3\dfrac{3}{8}-|-0.75|+(-5\dfrac{1}{2})+\left|-2\dfrac{5}{8}\right|.$

(1)原式$=-8+15-9+12=-8-9+15+12=-17+27=10.$
(2)原式$=-\dfrac{6}{5}-7+\dfrac{16}{5}-1=-\dfrac{6}{5}+\dfrac{16}{5}-7-1=2-7-1=-6.$
(3)原式$=-11\dfrac{2}{3}+7\dfrac{2}{5}-12\dfrac{1}{3}+4\dfrac{1}{5}=-11\dfrac{2}{3}-12\dfrac{1}{3}+(7\dfrac{2}{5}+4\dfrac{1}{5})=-24+11\dfrac{3}{5}=-12\dfrac{2}{5}.$
(4)原式$=2\dfrac{3}{5}-1\dfrac{1}{2}+3\dfrac{3}{10}+2\dfrac{1}{2}=2\dfrac{6}{10}+3\dfrac{3}{10}+(2\dfrac{1}{2}-1\dfrac{1}{2})=5\dfrac{9}{10}+1=6\dfrac{9}{10}.$
(5)原式$=\dfrac{3}{4}+3\dfrac{3}{8}-\dfrac{3}{4}-5\dfrac{1}{2}+2\dfrac{5}{8}=\dfrac{3}{4}-\dfrac{3}{4}+3\dfrac{3}{8}+2\dfrac{5}{8}-5\dfrac{1}{2}=6-5\dfrac{1}{2}=\dfrac{1}{2}.$

4. 计算：
(1)$49 - (-20.6) - \dfrac{3}{5}$；
(2)$-5.27 + 3.8 - (-1.2) + (-0.5) - 0.73$；
(3)$-\dfrac{4}{5} - \dfrac{2}{3} + \dfrac{3}{5} - \dfrac{1}{3} + \dfrac{2}{5}$；
(4)$(-4\dfrac{7}{8}) - (-5\dfrac{1}{2}) + (-4\dfrac{1}{2}) - (+3\dfrac{1}{8})$；
(5)$(-1\dfrac{1}{2}) - 1\dfrac{1}{4} + (-2\dfrac{1}{2}) - (-3\dfrac{3}{4}) - (-1\dfrac{1}{4}) + 4.$

(1)原式$=49+20.6-0.6=69.$
(2)原式$=-5.27-0.73+3.8+1.2-0.5=-6+5-0.5=-1.5.$
(3)原式$=-\dfrac{4}{5}+\dfrac{3}{5}+\dfrac{2}{5}-(\dfrac{1}{3}+\dfrac{2}{3})=\dfrac{1}{5}-1=-\dfrac{4}{5}.$
(4)原式$=-4\dfrac{7}{8}+5\dfrac{1}{2}-4\dfrac{1}{2}-3\dfrac{1}{8}=(-4\dfrac{7}{8}-3\dfrac{1}{8})+(5\dfrac{1}{2}-4\dfrac{1}{2})=-8+1=-7.$
(5)原式$=-1\dfrac{1}{2}-2\dfrac{1}{2}-1\dfrac{1}{4}+1\dfrac{1}{4}+3\dfrac{3}{4}+4=-4+3\dfrac{3}{4}+4=3\dfrac{3}{4}.$